guile: Add as_a_scm_t_subr

Building GDB in C++ mode on Fedora 20, the gdb/guile/ code shows ~280
errors like:

 src/gdb/guile/guile.c:515:1: error: invalid conversion from ‘scm_unused_struct* (*)(SCM, SCM) {aka scm_unused_struct* (*)(scm_unused_struct*, scm_unused_struct*)}’ to ‘scm_t_subr {aka void*}’ [-fpermissive]

This commit fixes them all.

gdb/ChangeLog:
2015-09-07  Pedro Alves  <palves@redhat.com>

	* guile/guile-internal.h (as_a_scm_t_subr): New.
	* guile/guile.c (misc_guile_functions): Use it.
	* guile/scm-arch.c (arch_functions): Use it.
	* guile/scm-block.c (block_functions, gdbscm_initialize_blocks):
	Use it.
	* guile/scm-breakpoint.c (breakpoint_functions): Use it.
	* guile/scm-cmd.c (command_functions): Use it.
	* guile/scm-disasm.c (disasm_functions): Use it.
	* guile/scm-exception.c (exception_functions)
	(private_exception_functions): Use it.
	* guile/scm-frame.c (frame_functions)
	* guile/scm-gsmob.c (gsmob_functions): Use it.
	* guile/scm-iterator.c (iterator_functions): Use it.
	* guile/scm-lazy-string.c (lazy_string_functions): Use it.
	* guile/scm-math.c (math_functions): Use it.
	* guile/scm-objfile.c (objfile_functions): Use it.
	* guile/scm-param.c (parameter_functions): Use it.
	* guile/scm-ports.c (port_functions, private_port_functions): Use
	it.
	* guile/scm-pretty-print.c (pretty_printer_functions): Use it.
	* guile/scm-progspace.c (pspace_functions): Use it.
	* guile/scm-string.c (string_functions): Use it.
	* guile/scm-symbol.c (symbol_functions): Use it.
	* guile/scm-symtab.c (symtab_functions): Use it.
	* guile/scm-type.c (type_functions, gdbscm_initialize_types): Use
	it.
	* guile/scm-value.c (value_functions): Use it.

gdb/guile/scm-string.c

@@ -268,7 +268,7 @@ gdbscm_string_to_argv (SCM string_scm)
 
 static const scheme_function string_functions[] =
 {
-  { "string->argv", 1, 0, 0, gdbscm_string_to_argv,
+  { "string->argv", 1, 0, 0, as_a_scm_t_subr (gdbscm_string_to_argv),
   "\
 Convert a string to a list of strings split up according to\n\
 gdb's argv parsing rules." },