fix debug build

Fix the following compile error when compiled with --enable-debug
flowgrind.c: In function ‘sanity_check’:
flowgrind.c:2871:2: error: ‘id’ undeclared (first use in this function)
flowgrind.c:2871:2: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [flowgrind-flowgrind.o] Error 1
make[2]: Leaving directory `/home/travis/build/flowgrind/flowgrind/src'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/travis/build/flowgrind/flowgrind'
make: *** [all] Error 2