Another OPENSSL_NO_THREADS build fix.

GCC gets unhappy if we don't initialize the padding.

Change-Id: I084ffee1717d9025dcb10d8f32de0da2339c7f01
Reviewed-on: https://boringssl-review.googlesource.com/7797
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index a3f3230..ded43fe 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -366,7 +366,7 @@
 struct CRYPTO_STATIC_MUTEX {
   char padding;  /* Empty structs have different sizes in C and C++. */
 };
-#define CRYPTO_STATIC_MUTEX_INIT {}
+#define CRYPTO_STATIC_MUTEX_INIT { 0 }
 #elif defined(OPENSSL_WINDOWS)
 struct CRYPTO_STATIC_MUTEX {
   CRYPTO_once_t once;