x86/paravirt: Make BUG_func() usable by non-GPL modules

Several inlined functions subject to paravirt patching are referencing
BUG_func() after the recent switch to the alternative patching
mechanism.

As those functions can legally be used by non-GPL modules, BUG_func()
must be usable by those modules, too. So use EXPORT_SYMBOL() when
exporting BUG_func().

Fixes: 9824b00c ("x86/paravirt: Move some functions and defines to alternative.c")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240109082232.22657-1-jgross@suse.com