Turn off unused-function warning in GCC

It is off in upstream IREE code base

Change-Id: Ib94d038ae8d757d23efa73ddd324a0ad624d1668

cmake/sparrow_ops.cmake

diff --git a/cmake/sparrow_ops.cmake b/cmake/sparrow_ops.cmake
index 586ccf5..7310266 100644
--- a/cmake/sparrow_ops.cmake
+++ b/cmake/sparrow_ops.cmake
@@ -95,6 +95,7 @@
   "-Wall"
   # Disable warnings we don't care about or that generally have a low
   # signal/noise ratio.
+  "-Wno-unused-function"
 )