LinuxRPM/Makefile: fix backquote which was triggering recursive make

The following Makefile snippet:

--snip--
all:
	@echo "To build RPMS run `make glusterrpms`"
--snip--

was using `backquotes` in the string, and thereby triggering a nested
make on "glusterrpms" target implicitly as part of 'make all', thus
defeating the intention.

So use single quotes instead of backquotes.

It is unexplained why this nested 'make glusterrpms' was not called on
the build server during regression test.

Change-Id: I8a8d1d4f565d03cfb27341e6183af5b09530eb60
BUG: 819130
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4532
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>