Replace YYUSE call with void cast in src/parser.y
Replace YYUSE call with void cast in src/parser.y

The YYUSE macro was renamed to YY_USE in bison 3.7.5; we might as well
avoid using it altogether and cast the unused variable to void instead.

Fixes the following linker error:

/usr/bin/ld: kafel/libkafel.a(libkafel.o): in function `kafel_yyerror':
arm_syscalls.c:(.text+0x6984): undefined reference to `YYUSE'

google/kafel,google/kafel,google/kafel