pythonPackages.magic: Fix loading libmagic.so

Even with the previous `python.patch` file around, libmagic.so isn't
found. Instead patch it to use an absolute path, using a similar
approach that is used by the `python_magic` package (which is another
libmagic python wrapper library as well).

pkgs/tools/misc/file/python.patch

@@ -1,11 +0,0 @@
---- file-5.11/python/magic.py	2011-09-22 18:57:41.000000000 +0200
-+++ file-5.11/python/magic.py.1	2012-04-30 15:00:31.750213810 +0200
-@@ -13,7 +13,7 @@
-     Loads the shared library through ctypes and returns a library
-     L{ctypes.CDLL} instance 
-     """
--    return ctypes.cdll.LoadLibrary(find_library('magic'))
-+    return ctypes.CDLL('libmagic.so')
- 
- _libraries = {}
- _libraries['magic'] = _init()