Provide get_shell declaration in procfs.c

Provide get_shell declaration in procfs.c

The Solaris build is currently broken:

/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c: In member function ‘virtual void procfs_target::create_inferior(const char*, const string&, char**, int)’:
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c:3038:28: error: ‘get_shell’ was not declared in this scope
   const char *shell_file = get_shell ();
                            ^~~~~~~~~
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c:3038:28: note: suggested alternative: ‘getusershell’
   const char *shell_file = get_shell ();
                            ^~~~~~~~~
                            getusershell

The following patch fixes this.  Tested on amd64-pc-solaris2.11.

2018-10-29  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	* procfs.c: Include common/pathstuff.h.