audio/giada: fix build on armv7

This port bundles a copy of JUCE.

juce_BasicNativeHeaders.h pulls in <sys/user.h> which in turn pulls in
<machine/atomic.h> transitively, which then defines a mb macro polluting
the name space. <sys/user.h> is only needed for struct kproc_info,
which is only needed in juce_SystemStats_linux.cpp to check for
P_TRACED. Remove <sys/user.h> from juce_BasicNativeHeaders.h and only
include it in juce_SystemStats_linux.cpp to avoid namespace pollution
issues.

As always, please try to unbundle dependencies if possible.

Approved by: portmgr (build fix blanket)
MFH: 2026Q3
PR: 283492