shared/: Please compiler when passed certain compile flags

Fixes the following warn/error when using combination of flags like
building with debug, when disabling optimization and/or when enabling ASAN:

../shared/option-parser.c:61:1: error: control reaches end of non-void function [-Werror=return-type]

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
2 jobs for warn-compiler in 1 minute and 23 seconds (queued for 2 seconds)
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Status Job ID Name Coverage
  Container Prep
passed #477674
container_prep

00:00:26

 
  Build
passed #477675
build-native-meson

00:00:56