parser: fix Makefile hardcoded paths to flex and bison

Closes #4

See merge request apparmor/apparmor!224

Acked-by: Christian Boltz <apparmor@cboltz.de> for 2.10..master

(cherry picked from commit 34cf08503613a60298281200db9218a7532a1e31)

17e059a2 parser: fix Makefile hardcoded paths to flex and bison