build-sys: add a phony target for cppcheck

The cppcheck target was introduced by commit 16f4efb415
"build-sys: add cppcheck target". But it is preferable to use a make phony target
for it, as this patch does.

There are two general reasons to use a phony target: to avoid a
conflict with a file of the same name, and to improve performance. In
this case the first reason is obvious, and the second is that make
skips the implicit rule search for phony targets, since it knows that
phony targets do not name actual files that could be remade from other
files (as described in the "Gnu Make" Manual).

Makefile.am

@@ -5008,6 +5008,7 @@ tests += \
 	test-libsystemd-id128-sym
 endif
 
+.PHONY: cppcheck
 cppcheck:
 	cppcheck --enable=all -q $(top_srcdir)