sw_isr_table: Add braces around subobject

When building with clang, it warns:

subsys/mgmt/ec_host_cmd/backends/ec_host_cmd_backend_shi_npcx.c:1000:2:
error: suggest braces around initialization of subobject
[-Werror,-Wmissing-braces]

IRQ_CONNECT(DT_INST_IRQN(0), DT_INST_IRQ(0, priority), shi_npcx_isr,
            DEVICE_DT_INST_GET(0), 0);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/irq.h:49:2: note: expanded from macro 'IRQ_CONNECT'
ARCH_IRQ_CONNECT(irq_p, priority_p, isr_p, isr_param_p, flags_p)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/arch/arm/irq.h:124:2: note: expanded from macro
'ARCH_IRQ_CONNECT'
Z_ISR_DECLARE(irq_p, 0, isr_p, isr_param_p); \
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/sw_isr_table.h:227:2: note: expanded from macro
'Z_ISR_DECLARE'
Z_ISR_DECLARE_C(irq, flags, func, param, __COUNTER__)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0
to see all)
include/zephyr/sw_isr_table.h:218:16: note: expanded from macro
'_Z_ISR_DECLARE_C'
{irq, flags, _MK_ISR_ELEMENT_SECTION(counter)}
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/sw_isr_table.h:197:42: note: expanded from macro
'_MK_ISR_ELEMENT_SECTION'
#define _MK_ISR_ELEMENT_SECTION(counter) _MK_ISR_SECTION_NAME(irq,
                                         __FILE__, counter)
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/sw_isr_table.h:195:2: note: expanded from macro
'_MK_ISR_SECTION_NAME'
"." Z_STRINGIFY(prefix) "." file "." Z_STRINGIFY(counter)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Tom Hughes <tomhughes@chromium.org>