[libc] Added closing quote (#145101)

Error message was missing a closing quote, added it.

Co-authored-by: Sriya Pratipati <[email protected]>

Author: sribee8

libc/hdr/types/mbstate_t.h

@@ -15,7 +15,7 @@
 
 #else // Overlay mode
 
-#error "Cannot overlay mbstate_t
+#error "Cannot overlay mbstate_t"
 
 #endif // LLVM_LIBC_FULL_BUILD