contrib: speexdsp: fix NEON support

The HAVE_NEON flag is only ever defined on select platforms. That is to say first Android which had one build with and one without it. Later on, it was also added to iOS.

It cannot be assumed that NEON is not there just because the flag is unspecified. This silently broke NEON support on Linux, and literally any other platform at the time this regression was introduced.

Strictly speaking, we could keep checking the flag on Android and iOS only. But as of now, both platforms have HAVE_NEON always set, so this patch is a no-op in those cases.

Regression from ef98e81f.

This will need back-porting.