Add libicu manifest

Summary: Adding libicu manifest, which will be used by f4d/velox

Reviewed By: amitkdutta

Differential Revision: D25934666

fbshipit-source-id: 247d9961c90fcfc497498b556626e9ef1739b191

build/fbcode_builder/manifests/libicu

+[manifest]
+name = libicu
+
+[rpms]
+libicu-devel
+
+[debs]
+libicu-dev
+
+[download]
+url = https://github.com/unicode-org/icu/releases/download/release-68-2/icu4c-68_2-src.tgz
+sha256 = c79193dee3907a2199b8296a93b52c5cb74332c26f3d167269487680d479d625
+
+[build.not(os=windows)]
+builder = autoconf
+subdir = icu/source
+
+[build.os=windows]
+builder = nop