configure --enable-libmnl: Add libmnl to libosmocore.pc.in Requires

Change-Id: I8ad8418ada95e79bb6079f34c6b57817c6f6ab11
diff --git a/libosmocore.pc.in b/libosmocore.pc.in
index d804b8e..6479221 100644
--- a/libosmocore.pc.in
+++ b/libosmocore.pc.in
@@ -6,7 +6,7 @@
 Name: Osmocom Core Library
 Description: C Utility Library
 Version: @VERSION@
-Requires: talloc
+Requires: talloc @LIBMNL_PC@
 Requires.private: @LIBSCTP_PC@
 Libs: -L${libdir} -losmocore
 Libs.private: @PTHREAD_LIBS@ @LIBSCTP_LIBS@