Accomodate clang-cl in test common/127

clang-cl does support '-Wmissing-include-dirs' (unlike msvc), but doesn't
(currently) support '/ZI' (which is used by the default 'debug' buildtype),
the presence of which will cause an 'unknown-argument' warning, which is
treated as an error with 'Werror'.

Adjust the default buildtype so this test can pass with clang-cl

test cases/common/127 no buildincdir/meson.build

@@ -1,5 +1,5 @@
 project('nobuilddir', 'c',
-  default_options : 'werror=true')
+  default_options : ['werror=true', 'buildtype=plain'])
 
 cc = meson.get_compiler('c')