Fix procfs.c compilation

Fix procfs.c compilation

procfs.c doesn't compile on Solaris:

/vol/src/gnu/gdb/hg/master/local/gdb/procfs.c: In member function ‘virtual bool procfs_target::info_proc(const char*, info_proc_what)’:
/vol/src/gnu/gdb/hg/master/local/gdb/procfs.c:3302:3: error: ‘gdb_argv’ was not declared in this scope
 3302 |   gdb_argv built_argv (args);
      |   ^~~~~~~~
/vol/src/gnu/gdb/hg/master/local/gdb/procfs.c:3303:20: error: ‘built_argv’ was not declared in this scope; did you mean ‘buildargv’?
 3303 |   for (char *arg : built_argv)
      |                    ^~~~~~~~~~
      |                    buildargv

Fixed by including  "gdbsupport/buildargv.h".

Tested on amd64-pc-solaris2.11, sparcv9-sun-solaris2.11.