Add missing StringMap.h inclusion, apparently clangd is not covered by check-clang-tools zzz

Author: rnk

clang-tools-extra/clangd/FS.h

@@ -12,6 +12,7 @@
 #include "Path.h"
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/Support/VirtualFileSystem.h"
 
 namespace clang {