build: Add linphone.scm to gnu/local.mk.

This is a followup to d5a8e391.

* gnu/local.mk (GNU_SYSTEM_MODULES): Add linphone.scm.

gnu/local.mk

@@ -320,6 +320,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/packages/libusb.scm			\
   %D%/packages/libunwind.scm			\
   %D%/packages/lighting.scm                     \
+  %D%/packages/linphone.scm			\
   %D%/packages/linux.scm			\
   %D%/packages/lirc.scm				\
   %D%/packages/lisp.scm				\