[LibOS] Makefile.rules: Specify path to pal-symbols via simple assignment
Previously, Makefile.rules specified the absolute path to pal-symbols
file via recursive assignment ("="). However, because Makefile.rules
is included from other Makefiles, this led to incorrect path
("graphene/Pal/src/host/Linux/Pal/src/pal-symbols"). This commit
replaces recursive assignment "=" with simple only-once ":=".