Move non-virtual-dtor warning to C++ only

Causes a warning when building C files.

Change-Id: Ib82134a0bb7a15c88a3edfbdb04d570fde643198
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index 04b0243..3316607 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -27,13 +27,13 @@
   cflags = [
     "-Wall",
     "-Wextra",
-    "-Wnon-virtual-dtor",
 
     # Make all warnings errors, except for the exemptions below.
     "-Werror",
     "-Wno-error=cpp",  # preprocessor #warning statement
     "-Wno-error=deprecated-declarations",  # [[deprecated]] attribute
   ]
+  cflags_cc = [ "-Wnon-virtual-dtor" ]
 }
 
 config("cpp17") {