gdb: fix pid attach (#2474)

Author: Arusekk

pwnlib/gdb.py

@@ -1173,7 +1173,7 @@ def findexe():
         cmd += [exe]
 
     if pid and not context.os == 'android':
-        cmd += [str(pid)]
+        cmd += ['-p', str(pid)]
 
     if context.os == 'android' and pid:
         runner  = _get_runner()