[libc] pthread.h.def: add PTHREAD_MUTEX_INITIALIZER

Adds PTHREAD_MUTEX_INITIALIZER for compiler-rt's builtins/emutls.c

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D156276

Author: alfredfo

libc/include/pthread.h.def

@@ -13,6 +13,7 @@
 
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB
 
+#define PTHREAD_MUTEX_INITIALIZER {0}
 #define PTHREAD_ONCE_INIT {0}
 
 enum {