add extern C to libc-shim string.h in C++ mode

src/libc-shim/include/string.h

@@ -1,5 +1,9 @@
 #include "stb/stb_sprintf.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void* memset(void* b, int c, size_t n);
 void* memcpy(void* __restrict dst, const void* __restrict src, size_t n);
 void* memmove(void* dst, const void* src, size_t n);
@@ -12,3 +16,7 @@ char* strcpy(char* __restrict s1, const char* __restrict s2);
 
 #define snprintf stbsp_snprintf
 #define vsnprintf stbsp_vsnprintf
+
+#ifdef __cplusplus
+} // extern "C"
+#endif