mpfr: use patch-kernel to apply downloaded patch

Reported by Emmanuel Riou <riou.emmanuel@googlemail.com>.

With the recent change to use $(TOPDIR)/dl as default for $(DL_DIR)
(416323e9), DL_DIR is now a relative path (./dl), which breaks the
mpfr patch handling as it changes dir.

Fix is by using patch-kernel instead of calling patch, which does
the right thing.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>