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/libosmogsm.pc.in b/libosmogsm.pc.in
index 753bb3a..0160be8 100644
--- a/libosmogsm.pc.in
+++ b/libosmogsm.pc.in
@@ -6,6 +6,6 @@
 Name: Osmocom GSM Core Library
 Description: GSM Core Library
 Version: @VERSION@
-Libs: -L${libdir} -losmogsm
+Libs: -L${libdir} @TALLOC_LIBS@ -losmogsm -losmocore
 Cflags: -I${includedir}/