Bug 1112222 - remove moz_extensions.c from GENERATED_FILES in parser/expat/lib/; r=mshal

moz_extensions.c isn't generated by the build process; it lives as a
checked-in file and is #include'd by xmltok.c in the same directory.
Bug 1112608 - use GENERATED_INCLUDES in security/manager/{boot,pki}/src/; r=mshal

The sole use of Makefile.in in the security/manager/{boot,pki}/src/
directories is so we can add $(DIST)/public/nss to INCLUDES.
GENERATED_INCLUDES can be used to handle this case instead, at the cost
of hardcoding the path to $(DIST).  This seems reasonable enough, since
a number of moz.build files already know about dist/ and its location
within the objdir.