scons: don't add -Wmissing-field-initializers

scons: don't add -Wmissing-field-initializers

The warning is absolutely useless. It doesn't actually say that there are
uninitialized variables. It points out the fact that there are missing
initializers and that variables are initialized to zero implicitly, which is
exactly what we want and what we commonly make use of.

C90 and C99 require all unspecified variables in the initializer list to be set
to zero.