stageGain is uninitialized in RISC-V

[U2FsdGVkX1]

In RISC-V, when optimization is enabled, it will fail to build with
the following:
../src/ipa/libipa/exposure_mode_helper.cpp:210:52: error: ‘stageGain’
may be used uninitialized [-Werror=maybe-uninitialized]
  210 |         shutter = clampShutter(exposure / clampGain(stageGain));
      |                                           ~~~~~~~~~^~~~~~~~~~~
../src/ipa/libipa/exposure_mode_helper.cpp:169:16: note: ‘stageGain’
was declared here
  169 |         double stageGain;
      |                ^~~~~~~~~

I posted an issue on GitHub and need more people to test it
https://github.com/kbingham/libcamera/pull/95