build: Actually use the extra warnings

I forgot to remove the '-Werror=' part from all the extra warnings, so
the warning/error flags we generated were '-Werror=-Werror=warning-flag'
or 'W-Werror=warning-flag' - but because our compiler flag checking
infrastructure works so nicely, it just ignored these obviously wrong
flags.

Fixes commit 362e91c40b

meson.build

@@ -294,25 +294,25 @@ elif cc.get_id() == 'gcc' or cc.get_id() == 'clang'
   ]
   
   extra_warnings = [
-    '-Werror=address',
-    '-Werror=array-bounds',
-    '-Werror=empty-body',
-    '-Werror=implicit',
-    '-Werror=implicit-fallthrough',
-    '-Werror=init-self',
-    '-Werror=int-to-pointer-cast',
-    '-Werror=main',
-    '-Werror=missing-braces',
-    '-Werror=missing-declarations',
-    '-Werror=missing-prototypes',
-    '-Werror=nonnull',
-    '-Werror=pointer-to-int-cast',
-    '-Werror=redundant-decls',
-    '-Werror=return-type',
-    '-Werror=sequence-point',
-    '-Werror=trigraphs',
-    '-Werror=vla',
-    '-Werror=write-strings',
+    'address',
+    'array-bounds',
+    'empty-body',
+    'implicit',
+    'implicit-fallthrough',
+    'init-self',
+    'int-to-pointer-cast',
+    'main',
+    'missing-braces',
+    'missing-declarations',
+    'missing-prototypes',
+    'nonnull',
+    'pointer-to-int-cast',
+    'redundant-decls',
+    'return-type',
+    'sequence-point',
+    'trigraphs',
+    'vla',
+    'write-strings',
   ]
 
   if get_option('buildtype').startswith('debug')