setup.py: fix bug related to BORG_LZ4_PREFIX processing

setup.py

@@ -111,7 +111,7 @@ library_dirs.append(os.path.join(ssl_prefix, 'lib'))
 
 possible_lz4_prefixes = ['/usr', '/usr/local', '/usr/local/opt/lz4', '/usr/local/lz4', '/usr/local/borg', '/opt/local']
 if os.environ.get('BORG_LZ4_PREFIX'):
-    possible_openssl_prefixes.insert(0, os.environ.get('BORG_LZ4_PREFIX'))
+    possible_lz4_prefixes.insert(0, os.environ.get('BORG_LZ4_PREFIX'))
 lz4_prefix = detect_lz4(possible_lz4_prefixes)
 if lz4_prefix:
     include_dirs.append(os.path.join(lz4_prefix, 'include'))