fix(embed): define TSRMLS cache for ZTS builds

Author: Qard

src/embed/embed.c

@@ -1,5 +1,9 @@
 #include "embed.h"
 
+#ifdef ZTS
+ZEND_TSRMLS_CACHE_DEFINE()
+#endif
+
 // We actually use the PHP embed API to run PHP code in test
 // At some point we might want to use our own SAPI to do that
 void* ext_php_rs_embed_callback(int argc, char** argv, void* (*callback)(void *), void *ctx) {