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/libosmousb.pc.in b/libosmousb.pc.in
index ce6d271..dbedb0e 100644
--- a/libosmousb.pc.in
+++ b/libosmousb.pc.in
@@ -6,6 +6,6 @@
 Name: Osmocom libusb (USB) integration
 Description: C Utility Library
 Version: @VERSION@
-Libs: -L${libdir} @TALLOC_LIBS@ -losmousb -losmocore
+Requires: talloc, libusb-1.0, libosmocore
+Libs: -L${libdir} -losmousb
 Cflags: -I${includedir}/
-