[build] Add src/closures.c

Needed by the no-opt debug build.

Change-Id: I8a0ff08e947a29370a75a6daaee74cc32b5095f2
diff --git a/BUILD.gn b/BUILD.gn
index dbd1845..5023a28 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -51,6 +51,8 @@
   cflags = [
     # Suppress warnings in upstream code that are triggered by Fuchsia compilation flags.
     "-Wno-conversion",
+    "-Wno-null-pointer-arithmetic",
+    "-Wno-sign-compare",
 
     # For asm keyword in src/aarch64/ffi.c
     "-std=gnu11",
@@ -68,6 +70,7 @@
   ]
 
   sources = [
+    "src/closures.c",
     "src/prep_cif.c",
     "src/types.c",
   ]