fbcode_builder: getdeps: fboss: add OpenBCM fetcher

Summary: As titled

Differential Revision: D18726457

fbshipit-source-id: 717a6cda4a78e1b246f20328df1f01b78560cac5

build/fbcode_builder/manifests/OpenBCM

+[manifest]
+name = OpenBCM
+
+[git]
+repo_url = https://github.com/Broadcom-Network-Switching-Software/OpenBCM.git
+
+[build]
+builder = nop
+
+[install.files]
+lib/x86-64 = lib
+include = include