src/boot/efi/meson.build: compile with -Werror

The src/boot/efi/ code is built separately from the rest of the meson
build, and doesn't pick up the meson --werror param added to the
top-level meson.build file.  This adds the cc -Werror param manually
to the cc builds of the EFI code.

src/boot/efi/meson.build

@@ -111,6 +111,7 @@ if have_gnu_efi
 
         compile_args = ['-Wall',
                         '-Wextra',
+                        '-Werror',
                         '-std=gnu90',
                         '-nostdinc',
                         '-ggdb', '-O0',