llext_link_helper.py: use a raw string for '\.so$' regex

Always use a "raw string" for regular expressions, otherwise the
backslash is used to start Unicode escape sequences.

Fixes hopefully harmless error:
```
scripts/llext_link_helper.py:38: SyntaxWarning: invalid escape sequence '\.'
  p = re.compile('(^lib|\.so$)')
```

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/llext_link_helper.py

@@ -35,7 +35,7 @@ def main():
 	elf = ELFFile(open(args.file, 'rb'))
 
 	text_addr = int(args.text_addr, 0)
-	p = re.compile('(^lib|\.so$)')
+	p = re.compile(r'(^lib|\.so$)')
 	module = p.sub('', args.file)
 
 	if elf.num_segments() != 0: