Fix pkg-config generation.

The NET_LIBS  configuration variable  was not  being exapanded  in the
pkgpconfig template file. This caused  static builds against hamlib to
fail on Windows because the WinSock library was not referenced.

hamlib.pc.in

@@ -10,4 +10,4 @@ Version: @PACKAGE_VERSION@
 Requires.private: @LIBUSB@
 Cflags: -I${includedir} @PTHREAD_CFLAGS@
 Libs: -L${libdir} -lhamlib
-Libs.private: @MATH_LIBS@ @DL_LIBS@ @PTHREAD_LIBS@
+Libs.private: @MATH_LIBS@ @DL_LIBS@ @NET_LIBS@ @PTHREAD_LIBS@