util/crossgcc: Bump MPFR version to 3.1.3

The current MPFR version contains a stale config.guess file
that requires special handling on ppc64el systems.  Bump
the MPFR version to the latest release.

Change-Id: I5e86c732c09f8a6a43f9812452124d64d337ea3f
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: https://review.coreboot.org/12832
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index 31db9cb..d88f130 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -32,7 +32,7 @@
 
 # GCC toolchain version numbers
 GMP_VERSION=6.1.0
-MPFR_VERSION=3.1.2
+MPFR_VERSION=3.1.3
 MPC_VERSION=1.0.3
 LIBELF_VERSION=0.8.13
 GCC_VERSION=5.2.0