Remove -Wno-unknown-pragmas from fusl build

fusl/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("fusl-x86_64") {
   configs = []
 
   asmflags = [ "-Wno-unused-command-line-argument" ]
 
   # Arch specific sources.
@@ skipping 67 matching lines @@
     "-fdata-sections",
     "-Werror=implicit-function-declaration",
     "-Werror=implicit-int",
     "-Werror=pointer-sign",
     "-Werror=pointer-arith",
 
     # Silent builds.
     "-Wno-ignored-attributes",
     "-Wno-string-plus-int",
     "-Wno-tautological-compare",
-    "-Wno-unknown-pragmas",
   ]
 
   # Arch specific includes.
   include_dirs = [ "arch/x86_64" ]
 
   # General includes.
   include_dirs += [
     "src/internal",
     "include",
   ]
@@ skipping 1286 matching lines @@
     "//fusl/src/unistd/ttyname.c",
     "//fusl/src/unistd/ttyname_r.c",
     "//fusl/src/unistd/ualarm.c",
     "//fusl/src/unistd/unlink.c",
     "//fusl/src/unistd/unlinkat.c",
     "//fusl/src/unistd/usleep.c",
     "//fusl/src/unistd/write.c",
     "//fusl/src/unistd/writev.c",
   ]
 }