Restore FFI-ability of struct mg_str

mgos_dlsym needs to include mg_str for ffi_exports to be able to use it

PUBLISHED_FROM=a2dd167c8e8f9e9b7316a856f19a4603080c3632

mongoose.c

@@ -1636,6 +1636,7 @@ void mbuf_remove(struct mbuf *mb, size_t n) {
 
 /* Amalgamated: #include "common/mg_mem.h" */
 /* Amalgamated: #include "common/mg_str.h" */
+/* Amalgamated: #include "common/platform.h" */
 
 #include <stdlib.h>
 #include <string.h>

mongoose.h

@@ -2095,8 +2095,6 @@ double cs_timegm(const struct tm *tm);
 
 #include <stddef.h>
 
-/* Amalgamated: #include "common/platform.h" */
-
 #ifdef __cplusplus
 extern "C" {
 #endif