libavutil/log.c: only include valgrind header when used.

This is cleaner, but it is also a workaround for when
the header exists, but cannot be compiled.
This will happen when the compiler has no inline asm
support.
Possibly the configure check should be improved as well.

libavutil/log.c

@@ -47,7 +47,7 @@ static AVMutex mutex = AV_MUTEX_INITIALIZER;
 
 #define LINE_SZ 1024
 
-#if HAVE_VALGRIND_VALGRIND_H
+#if HAVE_VALGRIND_VALGRIND_H && CONFIG_VALGRIND_BACKTRACE
 #include <valgrind/valgrind.h>
 /* this is the log level at which valgrind will output a full backtrace */
 #define BACKTRACE_LOGLEVEL AV_LOG_ERROR