[gdb/testsuite] Fix local-static.exp with gcc-4.8

With gdb.cp/local-static.exp and gcc 4.8, I see:
...
gdb compile failed, src/gdb/testsuite/gdb.cp/local-static.c: In function 'main':
src/gdb/testsuite/gdb.cp/local-static.c:148:3: error: 'for' loop initial \
  declarations are only allowed in C99 mode
   for (int i = 0; i < 1000; i++)
   ^
src/gdb/testsuite/gdb.cp/local-static.c:148:3: note: use option -std=c99 or \
  -std=gnu99 to compile your code
UNTESTED: gdb.cp/local-static.exp: c: failed to prepare
...

Fix this by moving the declaration of int i out of the for loop.

gdb/testsuite/ChangeLog:

2019-10-04  Tom de Vries  <tdevries@suse.de>

	* gdb.cp/local-static.c (main): Move declaration of int i out of the
	for loop.