Add the suggested <locale> header to fix building
with latest libc++ after https://reviews.llvm.org/D144331.

diff --git a/include/safe-ctype.h b/include/safe-ctype.h
index 4dcf849..01ee603 100644
--- a/include/safe-ctype.h
+++ b/include/safe-ctype.h
@@ -120,6 +120,9 @@ extern const unsigned char  _sch_tolower[256];
    So we include ctype.h here and then immediately redefine its macros.  */
 
 #include <ctype.h>
+#ifdef __cplusplus
+#include <locale>
+#endif
 #undef isalpha
 #define isalpha(c) do_not_use_isalpha_with_safe_ctype
 #undef isalnum