contrib/rpm: disable NDEBUG for meson builds

For better or worse, our release builds commonly do not disable assertions.
That means,

 - NDEBUG is not set, and assert() is in effect
 - G_DISABLE_ASSERT is not set, and g_assert() is in effect
 - G_DISABLE_CHECKS is not set, and g_return*() is in effect.

On the other hand, NM_MORE_ASSERTS is not enabled by default and nm_assert()
is stripped away. That is the actual purpose of nm_assert(): it is
commonly disabled on release builds, while all other assertions are
enabled.

Note that it is fully supported to build NetworkManager with all kind of
assertions disabled. However, such a configuration is not much tested
and I would not recommend it for that reason.

%meson expands to

  $ /usr/bin/meson --buildtype=plain --prefix=/usr --libdir=/usr/lib64 --libexecdir=/usr/libexec --bindir=/usr/bin --sbindir=/usr/sbin --includedir=/usr/include --datadir=/usr/share --mandir=/usr/share/man --infodir=/usr/share/info --localedir=/usr/share/locale --sysconfdir=/etc --localstatedir=/var --sharedstatedir=/var/lib --wrap-mode=nodownload --auto-features=enabled -Db_ndebug=true . x86_64-redhat-linux-gnu $OTHER_ARGS

thus passing -DNDEBUG to the meson build. Override that.

contrib/fedora/rpm/NetworkManager.spec

@@ -475,6 +475,7 @@ by nm-connection-editor and nm-applet in a non-graphical environment.
 %build
 %if %{with meson}
 %meson \
+	-Db_ndebug=false \
 	--warnlevel 2 \
 %if %{with test}
 	--werror \