Fix memcheck/tests/linux/dlclose_leak.c build under -std=gnu90.

Older gcc (4.8) default to GNU C90. Causing:

dlclose_leak.c:14:5: error: ‘for’ loop initial declarations are only
                     allowed in C99 mode

Fix by declaring int i before the loop.

memcheck/tests/linux/dlclose_leak.c

@@ -11,7 +11,8 @@ char* (*alloc_1_byte)(void);
 
 int main(int argc, char** argv)
 {
-    for (int i = 0; i < 2; ++i)
+    int i;
+    for (i = 0; i < 2; ++i)
     {
         char* memToLeak;
         char x __attribute__((unused));