missing.h: remove be32toh wrapper

missing.h: remove be32toh wrapper

As the inline comment says, it is applicable for older distributions
like RHEL 5. The extended support for RHEL 5 ended in 2020 (4 years ago)
so we're safe to drop this.

According to the Alpine build logs - the symbol has been present in musl
for years.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/86
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>