maint: avoid new syntax-check failure

maint: avoid new syntax-check failure

find-mount-point.h rightly includes <stdlib.h> for its use
of _GL_ATTRIBUTE_DEALLOC_FREE, which uses free, yet that new
inclusion provoked a syntax-check failure. Exempt this header
file as we've done for others.
* cfg.mk (exclude_file_name_regexp--sc_system_h_headers):
Add find-mount-point.h to the regexp.
(sc_system_h_headers): Use grep -E, for a more readable regexp.