Fixed ECPG regression test to make sure it uses absolute paths for include

files instead of relative ones which break vpath builds.

src/interfaces/ecpg/test/preproc/strings.pgc

-#include "../regression.h"
+#include <../regression.h>
 
 exec sql begin declare section;
-#include "strings.h"
+#include <strings.h>
 exec sql end declare section;
 
 int main(void)