gn2bp: Add `-Wno-null-pointer-subtraction` to the default flags

Test: m cronet_aml_components_cronet_android_cronet
Change-Id: I129160617c30b977a39400a91d10ab8a5e61cb8f
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 747c45a..3d98ce6 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -1209,6 +1209,7 @@
       '-Wno-sign-compare',
       '-Wno-sign-promo',
       '-Wno-unused-parameter',
+      '-Wno-null-pointer-subtraction', # Needed to libevent
       '-Wno-deprecated-non-prototype', # needed for zlib
       '-fvisibility=hidden',
       '-Wno-ambiguous-reversed-operator', # needed for icui18n