Add -Wno-ignored-pragmas.

* https://reviews.llvm.org/D90316 changed warning on
  FENV_ACCESS pragma from unknown to ignored.
  Existing -Wno-unknown-pragmas cannot suppress the new
  -Wignored-pragmas.

Bug: 178516148
Test: make with WITH_TIDY=1
Change-Id: I783feef35324ef43946efca844cd944410875bfa
diff --git a/libm/Android.bp b/libm/Android.bp
index 735b1cf..723d126 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -475,6 +475,7 @@
         "-fno-math-errno",
         "-Wall",
         "-Werror",
+        "-Wno-ignored-pragmas",
         "-Wno-missing-braces",
         "-Wno-parentheses",
         "-Wno-sign-compare",