build: Change MAKE_TARGETS in include/mk/lib.mk

Currently the lib.mk sets the MAKE_TARGETS to $(LIB) which does not
allow any other targets to be added before the lib.mk is included.

This commit changes the := to += so that the $(LIB) target is added to
the list instead.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>