Fix gdb procfs.c compilation on Solaris

Fix gdb procfs.c compilation on Solaris

Prompted by the creation of the gdb 8.0 branch, I tried to build it on
x86_64-pc-solaris2.12, but failed:

/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/procfs.c: In function `target_ops* procfs_target()':
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/procfs.c:186:27: error: invalid conversion from `void (*)(target_ops*, char*, char*, char**, int)' to `void (*)(target_ops*, const char*, const string&, char**, int) {aka void (*)(target_ops*, const char*, const std::__cxx11::basic_string<char>&, char**, int)}' [-fpermissive]
   t->to_create_inferior = procfs_create_inferior;
                           ^~~~~~~~~~~~~~~~~~~~~~
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/procfs.c: At global scope:
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/procfs.c:125:13: warning: `void procfs_create_inferior(target_ops*, char*, char*, char**, int)' declared `static' but never defined [-Wunused-function]
 static void procfs_create_inferior (struct target_ops *, char *,
             ^~~~~~~~~~~~~~~~~~~~~~
/vol/src/gnu/gdb/gdb-8.0-branch/local/gdb/procfs.c:4529:1: warning: `void procfs_create_inferior(target_ops*, const char*, const string&, char**, int)' defined but not used [-Wunused-function]
 procfs_create_inferior (struct target_ops *ops, const char *exec_file,
 ^~~~~~~~~~~~~~~~~~~~~~

This can easily be fixed by the following patch.

	* procfs.c (procfs_create_inferior): Change prototype to match
	definition.