Add extra includes required for Boost 1.75 and later.

Taken from upstram:

  https://github.com/ceph/ceph/commit/ebf3a0398f18eab67d2ba25e6a10b41ff140f6a4

diff --git a/src/rgw/rgw_string.h b/src/rgw/rgw_string.h
index 257daa9c1fe6e..90e64f98a2587 100644
--- a/src/rgw/rgw_string.h
+++ b/src/rgw/rgw_string.h
@@ -8,5 +8,7 @@
 #include <stdlib.h>
 #include <limits.h>
 #include <string_view>
+#include <string>
+#include <stdexcept>
 
 #include <boost/container/small_vector.hpp>
lass='main'><a href='/'>index</a> : <a href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='41e4c2d340ae173c8a4182209b12ed6e7dc46519'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>refs</a><a class='active' href='/guix/log/nix/boost'>log</a><a href='/guix/tree/nix/boost?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>tree</a><a href='/guix/commit/nix/boost?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>commit</a><a href='/guix/diff/nix/boost?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/nix/boost'>
<input type='hidden' name='id' value='41e4c2d340ae173c8a4182209b12ed6e7dc46519'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>root</a>/<a href='/guix/log/nix?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>nix</a>/<a href='/guix/log/nix/boost?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519'>boost</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/nix/boost?id=41e4c2d340ae173c8a4182209b12ed6e7dc46519&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-06-24 19:55:22 +0100'>2020-06-24</span></td><td><a href='/guix/commit/nix/boost?id=cd43b55da7ab51c996a7a3449c78f2a493616106'>nix: Tweak .gitignore files.</a><span class='msg-avail'>...<span class='msg-tooltip'>Remove .gitignore entries where they match source files that are tracked in
Git.  This is relevant to me at least, as some code searching tools use
.gitignore files and will ignore matched files.
</span></span></td><td>Christopher Baines</td></tr>