[mlir] Avoid including <alloca.h> on FreeBSD and NetBSD

Instead, include `<cstdlib>` which is the canonical header containing
the declaration of `alloca()`.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D107699

mlir/lib/ExecutionEngine/CRunnerUtils.cpp

@@ -15,7 +15,11 @@
 #include "mlir/ExecutionEngine/CRunnerUtils.h"
 
 #ifndef _WIN32
+#if defined(__FreeBSD__) || defined(__NetBSD__)
+#include <cstdlib>
+#else
 #include <alloca.h>
+#endif
 #include <sys/time.h>
 #else
 #include "malloc.h"