tftpd: tests: raise targeted cstd to c11

r358556 added alignas() use to the functional tests, which isn't defined
until C11. Raise the -std to C11 to fix the build under freebsd-gcc{6,9}.

Reported by: mhorne, Jenkins/CI

(cherry picked from commit 924e10b809a9fcbc8688c1b5848f60b48e6103fe)