xfrmi: Only build if libcharon is built

The kernel-netlink plugin is only built if libcharon is.

Closes strongswan/strongswan#167.

src/Makefile.am

@@ -42,6 +42,9 @@ endif
 
 if USE_LIBCHARON
   SUBDIRS += libcharon
+if USE_KERNEL_NETLINK
+  SUBDIRS += xfrmi
+endif
 endif
 
 if USE_FILE_CONFIG
@@ -143,7 +146,3 @@ endif
 if USE_TPM
   SUBDIRS += tpm_extendpcr
 endif
-
-if USE_KERNEL_NETLINK
-  SUBDIRS += xfrmi
-endif