[lit][REQUIRES] Fix llvm-debuginfod.test so it will run

The test was added in D114846 but missed one place to introduce the
'httplib' feature keyword, so it has been UNSUPPORTED everywhere.

Differential Revision: https://reviews.llvm.org/D136613

llvm/test/lit.cfg.py

@@ -456,6 +456,9 @@ def have_ld64_plugin_support():
 if config.have_curl:
     config.available_features.add('curl')
 
+if config.have_httplib:
+    config.available_features.add('httplib')
+
 if config.have_opt_viewer_modules:
     config.available_features.add('have_opt_viewer_modules')