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/libosmogb.pc.in b/libosmogb.pc.in
index a163cc4..d545ce2 100644
--- a/libosmogb.pc.in
+++ b/libosmogb.pc.in
@@ -6,6 +6,6 @@
 Name: Osmocom GPRS Gb Library
 Description: Osmocom GPRS Gb Interface (NS/BSSGP) Library
 Version: @VERSION@
-Libs: -L${libdir} @TALLOC_LIBS@ -losmogb -losmovty -losmocore
+Requires: libosmocore, libosmovty
+Libs: -L${libdir} -losmogb
 Cflags: -I${includedir}/ -fno-strict-aliasing
-