Fix build for systems without error().
* gnulib/error.c: New file (imported from Gnulib).
* gnulib/error.h: Likewise.
* conf/common.rmk (grub_mkisofs_SOURCES): Add `gnulib/error.c'.
* util/mkisofs/mkisofs.c (program_name): Remove `static' qualifier
(this variable is now used by error()).