math/openblas: Fix PLIST when INTERFACE64 option is enabled

from Makefile.install:
165 #Generating openblas.pc
166 ifeq ($(INTERFACE64),1)
167 SUFFIX64=64
168 endif
169 PKGFILE="$(DESTDIR)$(OPENBLAS_PKGCONFIG_DIR)/$(LIBSONAMEBASE)$(SUFFIX64).pc"
...
178 @cat openblas.pc.in >> "$(PKGFILE)"

PR: 275802
Submitted by: Sergey V. Koupreyenko <sergey.koupreyenko@gmail.com>
Approved by: Eijiro Shibusawa <phd_kimberlite@yahoo.co.jp> (maintainer)