meson: move service file into its own subdir

As a part of the overall movement of phosphor-state-manager to meson,
follow the standard set in other repos and put the service files in a
subdirectory. This commit does a small refactor to the existing automake
infrastructure. Future commits will utilize this in meson.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I63360b147d17788778c0cc8cb89e44a812ea49e8
diff --git a/Makefile.am b/Makefile.am
index 99cabb1..fc8f1bb 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -9,7 +9,7 @@
 endif
 
 systemdsystemunit_DATA = \
-  phosphor-systemd-target-monitor.service
+  ./service_files/phosphor-systemd-target-monitor.service
 
 AM_LIBS = $(CODE_COVERAGE_LIBS)
 AM_CPPFLAGS = $(CODE_COVERAGE_CPPFLAGS) -UNDEBUG $(GTEST_CPPFLAGS)