qnx: make alloca config test pass on QNX 7

Test failed on QNX 7, even though alloca is available. On QNX7,
it's a macro that expands to a line with NULL, but without define
for it.

alloca.cpp:44:5: error: 'NULL' was not declared in this scope

Task-number: QTBUG-59700
Change-Id: I3631d139990020a3adbab8b72e49929b6e721e80
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

config.tests/common/alloca/alloca.cpp

@@ -39,6 +39,9 @@
 
 #include <alloca.h>
 
+// extra include needed in QNX7 to define NULL for the alloca() macro
+#include <stdlib.h>
+
 int main(int, char **)
 {
     alloca(1);