tests/28 multiline string: include required headers

we're relying on printf being defined with stdio.h, which happens to
work sometimes. It doesn't work with Apple clang though.

test cases/common/28 multiline string/meson.build

@@ -26,6 +26,8 @@ endif
 
 cc = meson.get_compiler('c')
 prog = '''
+#include <stdio.h>
+
 int main(void) {
   int num = 1;
   printf("%d\n", num);