meson: re-enable -Wmaybe-uninitialized on -O[02] with recent gcc versions

meson: re-enable -Wmaybe-uninitialized on -O[02] with recent gcc versions

The warning was disabled in 8794164fed5f0142c34358613f92f4f761af4edd to avoid
false positives. But it is useful in finding errors, even if it sometimes
results in untrue warnings (c.f. 77fac974fe, da46a1bc3c).

After #19168, #19169, and #19175, there are no warnings with
-Dbuildtype=debug-optimized/-O2 and gcc-11.0.1-0.3.fc34.x86_64. Warnings
are reenabled for -O[23]

-O0 is good for development, and -O2 is the default optimization level for
Fedora package builds. -Os, -O3, -O1, and -Og still generate some warnings. In
fact, with -Os the number of warnings seems completely hopeless. Dozens and
dozens.