bfd BLD-POTFILES.in dependencies

A file that consists of a list of files doesn't depend on those files
being built.  This patch came from trying to avoid a maintainer-mode
make -j bug, where the recipe for targmatch.h was being run twice in
parallel.  Typical output shown below.

make[2]: Entering directory '/build/gas/all/bfd'
  GEN      bfdver.h
  GEN      elf32-target.h
  GEN      elf64-target.h
  GEN      targmatch.h
Making info in po
make[3]: Entering directory '/build/gas/all/bfd/po'
cd .. && make po/SRC-POTFILES.in
cd .. && make po/BLD-POTFILES.in
make[4]: Entering directory '/build/gas/all/bfd'
  GEN      elf32-aarch64.c
  GEN      elf64-aarch64.c
  GEN      elf32-ia64.c
  GEN      elf64-ia64.c
  GEN      elf32-loongarch.c
  GEN      elf64-loongarch.c
  GEN      elf32-riscv.c
  GEN      elf64-riscv.c
  GEN      peigen.c
  GEN      pepigen.c
  GEN      pex64igen.c
  GEN      pe-aarch64igen.c
  GEN      targmatch.h
make[4]: Entering directory '/build/gas/all/bfd'
  CCLD     doc/chew.stamp
mv: cannot stat 'targmatch.new': No such file or directory
make[4]: *** [Makefile:2325: targmatch.h] Error 1

	* Makefile.am (po/BLD-POTFILES.in): Don't depend on $(BLD_POTFILES).
	(po/SRC-POTFILES.in): Don't depend on $(SRC_POTFILES).