Fix FBString's new includes

Summary:
The headers <ext/hash*> were being included but not used when
_LIBSTDCXX_FBSTRING is defined. They have been relocated to within the
appropiate header block.

Test Plan:
fbconfig -r folly && fbmake runtests_opt

copy FBString and Malloc into libgcc, then
tp2_build libgcc/4.8.1/gcc-4.8.1-glibc-2.17-fb

Reviewed By: pgriess@fb.com

FB internal diff: D1187345

folly/FBString.h

@@ -73,11 +73,6 @@
 #include <bits/c++config.h>
 #endif
 
-#ifdef _GLIBCXX_SYMVER
-#include <ext/hash_set>
-#include <ext/hash_map>
-#endif
-
 #ifdef _LIBSTDCXX_FBSTRING
 
 #pragma GCC system_header
@@ -102,6 +97,11 @@
 #include "folly/Malloc.h"
 #include "folly/Hash.h"
 
+#ifdef _GLIBCXX_SYMVER
+#include <ext/hash_set>
+#include <ext/hash_map>
+#endif
+
 #endif
 
 // We defined these here rather than including Likely.h to avoid