cli: use libgit2 system includes

libgit2 may adjust the system includes - for example, to locate a
dependency installed in a funny location. Ensure that the cli
understands those include paths as well.

src/cli/CMakeLists.txt

@@ -4,7 +4,8 @@ set(CLI_INCLUDES
 	"${libgit2_SOURCE_DIR}/src/util"
 	"${libgit2_SOURCE_DIR}/src/cli"
 	"${libgit2_SOURCE_DIR}/include"
-	"${LIBGIT2_DEPENDENCY_INCLUDES}")
+	"${LIBGIT2_DEPENDENCY_INCLUDES}"
+	"${LIBGIT2_SYSTEM_INCLUDES}")
 
 if(WIN32 AND NOT CYGWIN)
 	file(GLOB CLI_SRC_OS win32/*.c)