libssh2.pc.in: fix the output of pkg-config --libs

... such that it does not include LDFLAGS used to build libssh2 itself.
There was a similar fix in the curl project long time ago:

https://github.com/bagder/curl/commit/curl-7_19_7-56-g4c8adc8

Bug: https://bugzilla.redhat.com/1279966
Signed-off-by: Kamil Dudka <kdudka@redhat.com>

libssh2.pc.in

@@ -12,6 +12,6 @@ URL: http://www.libssh2.org/
 Description: Library for SSH-based communication
 Version: @LIBSSH2VER@
 Requires.private: @LIBSREQUIRED@
-Libs: -L${libdir} -lssh2 @LDFLAGS@ @LIBS@
+Libs: -L${libdir} -lssh2 @LIBS@
 Libs.private: @LIBS@
 Cflags: -I${includedir}