security/libgcrypt: fix build with GCC on 8.x/amd64, 2nd attempt

Thanks to tijl@ for spotting the Makefile bug.

PR:		192555
Submitted by:	Carlos Jacobo Puga Medina <cpm@fbsd.es>
Approved by:	maintainer (timeout)


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@367293 35697150-7ecd-e111-bb59-0022644237b5
Fix misleading option name: the option uses plotutils, not gnuplot
No functional changes

PR:		193317
Submitted by:	amdmi3


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@367294 35697150-7ecd-e111-bb59-0022644237b5