Switch to use Requires.private for devmapper.pc and lvm2app.pc

Use Requires.private: instead of Libs.private:
Use UDEV_PC and SELINUX_PC for Require.private:

It looks like usage of Requires.private is prefered from Libs.private.
However pkg-config documentation is really poor here. But here is
short outcome:

There is a difference in Libs.private: and Requires.private: where
we specify libselinux instead of  -lselinux -lsepol.

We leave resolving of query like 'pkg-config --libs --static devmapper'
on taking proper selinux and udev libs to their .pc files instead of
hardcoding them into our .pc file which is might give incorrect answer.
- i.e. dependency of libselinux package might change and we may return
wrong list of linked libraries.

http://bugs.freedesktop.org/show_bug.cgi?id=4738
http://err.no/personal/blog/tech/2008-03-25-18-07_pkg-config,_sonames_and_Requires.private

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.65 - 
 =================================
+  Switch usage of Libs.private: to Requires.private: in devmapper.pc, lvm2app.pc.
   Use field Requires.private: for devmapper-event.pc.
   Add devmapper library to linked libdevmapper-event.so.
   Link liblvm2cmd.so with devmapper-event and devmapper libs.

libdm/libdevmapper.pc.in

@@ -8,4 +8,4 @@ Description: device-mapper library
 Version: @DM_LIB_PATCHLEVEL@
 Cflags: -I${includedir} 
 Libs: -L${libdir} -ldevmapper
-Libs.private: @SELINUX_LIBS@ @UDEV_LIBS@
+Requires.private: @SELINUX_PC@ @UDEV_PC@

liblvm/liblvm2app.pc.in

@@ -8,4 +8,4 @@ Description: lvm2 application library
 Version: @LVM_MAJOR@.@LVM_LIBAPI@
 Cflags: -I${includedir} 
 Libs: -L${libdir} -llvm2app
-Libs.private: -ldevmapper
+Requires.private: devmapper