remove our internal copy of talloc, use system libtalloc

Shipping our own private copy of talloc was a good idea in 2008,
when it was not readily available on most target platforms.  Today,
the situation is quite different, as it is a standard library on
major Linux distributions.
diff --git a/libosmogb.pc.in b/libosmogb.pc.in
index 33cacaf..a163cc4 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${libdir} -losmogb -losmovty
+Libs: -L${libdir} @TALLOC_LIBS@ -losmogb -losmovty -losmocore
 Cflags: -I${includedir}/ -fno-strict-aliasing