CI: Github Actions: re-enable Opentracing

Miroslav already fixed the build of OpenTracing so we can re-enable it
in the CI. For now variables are disabled but this will change soon.

.github/matrix.py

@@ -73,6 +73,7 @@ for CC in ["gcc", "clang"]:
             "CC": CC,
             "FLAGS": [
                 "USE_ZLIB=1",
+                "USE_OT=1",
                 "OT_INC=${HOME}/opt/include",
                 "OT_LIB=${HOME}/opt/lib",
                 "OT_RUNPATH=1",
@@ -147,6 +148,7 @@ matrix.append(
         "FLAGS": get_asan_flags(CC)
         + [
             "USE_ZLIB=1",
+            "USE_OT=1",
             "OT_INC=${HOME}/opt/include",
             "OT_LIB=${HOME}/opt/lib",
             "OT_RUNPATH=1",