clean-up pkg-config files: Make use of "Requires" as documented

According to the pkg-config manual, "Libs" should not contain flags
for _required_ packages.  Instead, they should be expressed via
"Requires".  Let's do that

Change-Id: I2ab1fe8e4bbfc120b471d6c9f2312a89dbc7d42b
diff --git a/libosmoctrl.pc.in b/libosmoctrl.pc.in
index 4676b31..e9947dc 100644
--- a/libosmoctrl.pc.in
+++ b/libosmoctrl.pc.in
@@ -6,6 +6,6 @@
 Name: Osmocom Control Interface Library
 Description: C Utility Library
 Version: @VERSION@
-Libs: -L${libdir} @TALLOC_LIBS@ -losmoctrl -losmogsm -losmocore
+Requires: talloc, libosmocore, libosmogsm
+Libs: -L${libdir} -losmoctrl
 Cflags: -I${includedir}/
-