Add extern "C" in libc-builtin-sysroot pthread.h (#933)

Add extern "C" in libc-builtin-sysroot pthread.h to fix mulit-thread
wasm app compiling error with C++ compiler

wamr-sdk/app/libc-builtin-sysroot/include/pthread.h

@@ -6,6 +6,10 @@
 #ifndef _WAMR_LIB_PTHREAD_H
 #define _WAMR_LIB_PTHREAD_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <stdint.h>
 
 /* Data type define of pthread, mutex, cond and key */
@@ -58,4 +62,8 @@ void *pthread_getspecific(pthread_key_t key);
 
 int pthread_key_delete(pthread_key_t key);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* end of _WAMR_LIB_PTHREAD_H */