include/Makefile: install systemd.h by default

Install systemd.h header file by default, as we have stub and defined
versions of the systemd functions for both non-systemd and systemd
enabled configurations.  This will help packagers to ship package
without systemd support (--enable-systemd=no).

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>

include/Makefile.am

@@ -2,8 +2,4 @@
 nobase_include_HEADERS = libcgroup.h libcgroup/error.h libcgroup/init.h \
 			 libcgroup/groups.h libcgroup/tasks.h \
 			 libcgroup/iterators.h libcgroup/config.h \
-			 libcgroup/log.h libcgroup/tools.h
-
-if WITH_SYSTEMD
-nobase_include_HEADERS += libcgroup/systemd.h
-endif
+			 libcgroup/log.h libcgroup/tools.h libcgroup/systemd.h