Fix installation of libgit2.pc file

configure_file outputs by default to binary dir,
and install(files) search from source dir.

CMakeLists.txt

@@ -111,7 +111,7 @@ INSTALL(TARGETS git2
     LIBRARY DESTINATION ${INSTALL_LIB}
     ARCHIVE DESTINATION ${INSTALL_LIB}
 )
-INSTALL(FILES libgit2.pc DESTINATION ${INSTALL_LIB}/pkgconfig )
+INSTALL(FILES ${CMAKE_BINARY_DIR}/libgit2.pc DESTINATION ${INSTALL_LIB}/pkgconfig )
 INSTALL(DIRECTORY include/git2 DESTINATION ${INSTALL_INC} )
 INSTALL(FILES include/git2.h DESTINATION ${INSTALL_INC} )