Improve librtlsdr.pc file

librtlsdr.pc should declare -lusb-1.0 in Libs.private section
to exclude usb library from dynamic linking.
References to libusb headers are not needed in Cflags, since these
headers are not used by external rtlsdr API, but this is optional.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=784912

librtlsdr.pc.in

@@ -6,6 +6,6 @@ includedir=@includedir@
 Name: RTL-SDR Library
 Description: C Utility Library
 Version: @VERSION@
-Cflags: -I${includedir}/ @RTLSDR_PC_CFLAGS@
-Libs: -L${libdir} -lrtlsdr -lusb-1.0
-Libs.private: @RTLSDR_PC_LIBS@
+Cflags: -I${includedir}/
+Libs: -L${libdir} -lrtlsdr
+Libs.private:  -lusb-1.0 @RTLSDR_PC_LIBS@