fixdep: fix CONFIG_IS_ENABLED etc. handling

When fixdep detects CONFIG_IS_ENABLED and other similar macros, it must
parse the macro parameter to determine which actual CONFIG_ option is
being referenced. This involves moving a pointer forward through the
entire CONFIG_ option "word". Currently, the code uses variable q to walk
through the word, but doesn't actually initialize it to point at the
parameter before doing so. Consequently, the walking process immediately
fails since it sees the macro invocatoins's ( rather than the expected
alpha-numeric characters in the macro parameter. Fix this by adding the
missing initialization.

Fixes: 67f2ee86 ("kbuild: fixdep: Resync this with v4.17")
Fixes: 7012865e ("gpio: fix test.py for gpio label lookup")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>