xdrgen: Make the xdrgen script location-independent

The @pythondir@ placeholder is meant for build-time substitution,
such as with autoconf. autoconf is not used in the kernel. Let's
replace that mechanism with one that better enables the xdrgen
script to be run from any directory.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

tools/net/sunrpc/xdrgen/xdrgen

@@ -10,8 +10,13 @@ __license__ = "GPL-2.0 only"
 __version__ = "0.2"
 
 import sys
+from pathlib import Path
 import argparse
 
+_XDRGEN_DIR = Path(__file__).resolve().parent
+if str(_XDRGEN_DIR) not in sys.path:
+    sys.path.insert(0, str(_XDRGEN_DIR))
+
 from subcmds import definitions
 from subcmds import declarations
 from subcmds import lint