tools/nolibc: tests: fix build on non-c99 compliant compilers

tools/nolibc: tests: fix build on non-c99 compliant compilers

Commit 9735716830f2 ("tools/nolibc: tests: add test for -fstack-protector")
brought a declaration inside the initialization statement of a for loop,
which breaks the build on compilers that do not default to c99
compatibility, making it more difficult to validate that the lib still
builds on such compilers. The fix is trivial, so let's move the
declaration to the variables block of the function instead. No backport
is needed.

Cc: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>