mtd-utils: serve_image: use proper POSIX_C_SOURCE value

struct addrinfo and friends conform to POSIX.1-2001, not earlier.
This patch fixes linking against latest glibc 2.22

Signed-off-by: Kirill Smirnov <kirill.k.smirnov@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
1 file changed