meson: Treat warnings as errors

Makefile treats warnings as errors, so enable the same option
for meson.

Makefile has an option to disable "warnings as errors" option
from command line. The same can be achieved with meson, see
scenario 2 below.

Tested: I verified the following scenarious

1) check that warnings treated as errors by enforcing higher
warning level
`meson setup --warnlevel=3 --wipe`
`ninja test`
shows tons of errors because warning level 3 is pedantic,
build fails

2) disable "warning as errors" option from command line
`meson configure -Dwerror=false`
`ninja test`
Now all warnings are non-blocking for build, so tests pass

3) back home
`meson configure -Dwerror=true`
`meson setup --warnlevel=2 --wipe`
`ninja test`
No warnings and tests pass

4) defaults: create new directory and run meson will all default
options
No warnings, all tests pass.

Change-Id: I06c72abf1c861f9b35736e02ef7f4a4fb6909b97
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/63852
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Original-Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72326
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/meson.build b/meson.build
index 3778f4d..db6f98b 100644
--- a/meson.build
+++ b/meson.build
@@ -5,6 +5,7 @@
   default_options : [
     'warning_level=2',
     'c_std=c99',
+    'werror=true',
     'optimization=s',
     'debug=false'],
 )