build: fix --disable-libsdbusplus

Since the --disable-libsdbusplus doesn't build the libsdbusplus
archive, we cannot link against it in example or test directories.
Disable those sub-directories if libsdbusplus is disabled.

Change-Id: I5a1441042d5b05f7bad9d840fea154270782bcbd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/Makefile.am b/Makefile.am
index 288b0a8..16a8b3d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -42,5 +42,7 @@
 pkgconfiglibdir = ${libdir}/pkgconfig
 pkgconfiglib_DATA = sdbusplus.pc
 
-endif
 SUBDIRS = . tools example test
+else
+SUBDIRS = . tools
+endif # WANT_LIBSDBUSPLUS