build: error out on -Wimplicit

These warnings are easy to miss during development:

    ../edid.c: In function ‘_di_edid_parse’:
    ../edid.c:254:25: warning: implicit declaration of function ‘di_edid_destroy’? [-Wimplicit-function-declaration]
      254 |                         di_edid_destroy(edid);
          |                         ^~~~~~~~~~~~~~~

And when missed, the code still compiles. Which means one needs to clean-build
to show them again.

Signed-off-by: Simon Ser <contact@emersion.fr>

meson.build

@@ -23,6 +23,8 @@ add_project_arguments(cc.get_supported_arguments([
 
 	'-Wno-unused-parameter',
 	'-Wno-missing-field-initializers',
+
+	'-Werror=implicit',
 ]), language: 'c')
 
 di_lib = library(