libavutil: Define the noreturn attribute for clang in MSVC mode as well

This is a follow-up to f637046d.

Without the noreturn attribute set, avconv_opt.c fails to build after
d2e6dd32 with the error "control may reach end of non-void function".
By making sure the noreturn attribute is set properly, this compiles
as intended.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/attributes.h

@@ -117,7 +117,7 @@
 #    define av_printf_format(fmtpos, attrpos)
 #endif
 
-#if AV_GCC_VERSION_AT_LEAST(2,5)
+#if AV_GCC_VERSION_AT_LEAST(2,5) || defined(__clang__)
 #    define av_noreturn __attribute__((noreturn))
 #else
 #    define av_noreturn