Add missing sentinel 'char *' casts in concat/reconcat calls

Add missing sentinel 'char *' casts in concat/reconcat calls

The wildebeest-debian-wheezy-i686 buildslave's build is broken due to:

 ../../binutils-gdb/gdb/python/python.c: In function void _initialize_python():
 ../../binutils-gdb/gdb/python/python.c:1709:36: error: missing sentinel in function call [-Werror=format]

Reproduced on Fedora 23 by sticking a few:

 #undef NULL
 #define 0

in build/gdb/build-gnulib/{stddef|signal|stdio}.h.  Hopefully this
caught all instances.

gdb/ChangeLog:
2016-04-21  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (try_open_dwop_file, open_dwo_file)
	(file_file_name, file_full_name): Add char * cast to sentinel in
	concat/reconcat calls.
	* event-top.c (top_level_prompt): Likewise.
	* guile/guile.c (initialize_scheme_side): Likewise.
	* linux-tdep.c (linux_fill_prpsinfo): Likewise.
	* macrotab.c (macro_source_fullname): Likewise.
	* main.c (get_init_files, captured_main): Likewise.
	* psymtab.c (psymtab_to_fullname): Likewise.
	* python/python.c (_initialize_python)
	(gdbpy_finish_initialization): Likewise.
	* source.c (symtab_to_fullname): Likewise.