Add c99_alloca.h include to fix compilation on Cygwin

Fix compilation on Cygwin, since 50b22354, by adding c99_alloca.h include,
which should know how to portably make the alloc() prototype available.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/ddebug/dd_util.h

@@ -33,6 +33,7 @@
 #include <unistd.h>
 #include <sys/stat.h>
 
+#include "c99_alloca.h"
 #include "os/os_process.h"
 #include "util/u_debug.h"