build: silence message in "tools/create-exports-NetworkManager.sh" about missing directory

When building with meson -Dppp=false, the following message is printed
during build:

  [623/671] Generating NetworkManager.ver with a custom command. find: ‘./src/ppp/’: No such file or directory

The message is harmless. Hide it.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/43

tools/create-exports-NetworkManager.sh

@@ -53,8 +53,8 @@ EOF
 
 get_symbols_missing() {
     (for f in $(find ./src/settings/plugins/*/${libs} \
-              ./src/devices/*/${libs} \
-              ./src/ppp/${libs} -name '*.so'); do
+                     ./src/devices/*/${libs} \
+                     ./src/ppp/${libs} -name '*.so' 2>/dev/null); do
         call_nm "$f" |
             sed -n 's/^\([U]\) \(\(nm_\|nmp_\|_nm\|NM\|_NM\|c_siphash_\).*\)$/\2/p'
     done) |