test: silence warnings on clang under clang-cl

When running under clang-cl mode, we do not define `__GNUC__`, resulting
in the test failing.

llvm-svn: 295560
diff --git a/libcxx/test/std/depr/depr.c.headers/stdio_h.pass.cpp b/libcxx/test/std/depr/depr.c.headers/stdio_h.pass.cpp
index 85f9d29..8670af6 100644
--- a/libcxx/test/std/depr/depr.c.headers/stdio_h.pass.cpp
+++ b/libcxx/test/std/depr/depr.c.headers/stdio_h.pass.cpp
@@ -99,7 +99,7 @@
 
 #include <cstdarg>
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic ignored "-Wformat-zero-length"
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations" // for tmpnam
 #endif