package/mc: fix typo BR2_PACKAGE_GMP -> BR2_PACKAGE_GPM

package/mc: fix typo BR2_PACKAGE_GMP -> BR2_PACKAGE_GPM

Commit 9cae8f557b9d3f18ccf3694155f452d6b55de5f6 introduced an optional
dependency on GPM, but got the name of the option wrong, and used
GMP. In fact, even the commit title was wrong.

This causes a build failure:

Makefile:578: *** gpm is in the dependency chain of mc that has added it to its _DEPENDENCIES variable without selecting it or depending on it from Config.in.  Stop.

Fixes:

  http://autobuild.buildroot.net/results/52fb92ae7dd55cba7d19862bb6cd89c80da9a4b6/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>