syslinux: fix invalid use of substituteInPlace

substituteInPlace was invoked with multiple targets on the command line, which
is not supported.

+2 -1
+2 -1
pkgs/os-specific/linux/syslinux/default.nix
··· 30 preBuild = '' 31 substituteInPlace Makefile --replace /bin/pwd $(type -P pwd) 32 substituteInPlace gpxe/src/Makefile.housekeeping --replace /bin/echo $(type -P echo) 33 - substituteInPlace utils/ppmtolss16 gpxe/src/Makefile --replace /usr/bin/perl $(type -P perl) 34 ''; 35 36 stripDebugList = "bin sbin share/syslinux/com32";
··· 30 preBuild = '' 31 substituteInPlace Makefile --replace /bin/pwd $(type -P pwd) 32 substituteInPlace gpxe/src/Makefile.housekeeping --replace /bin/echo $(type -P echo) 33 + substituteInPlace utils/ppmtolss16 --replace /usr/bin/perl $(type -P perl) 34 + substituteInPlace gpxe/src/Makefile --replace /usr/bin/perl $(type -P perl) 35 ''; 36 37 stripDebugList = "bin sbin share/syslinux/com32";