tests: interrupt: Fix armclang compiler warning

When building this test with the armclang compiler we get the following
warning:

tests/kernel/interrupt/src/dynamic_isr.c
tests/kernel/interrupt/src/dynamic_isr.c:23:32: error: 'used' attribute
ignored on a non-definition declaration [-Werror,-Wignored-attributes]

extern struct _isr_table_entry __sw_isr_table _sw_isr_table[];
                               ^
There is no need to add the __sw_isr_table on the extern, so remove it
to address the warning.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>