rtc: cmos: Fix non-ACPI undefined reference to `hpet_rtc_interrupt'

Fix a commit 311ee9c1 ("rtc: cmos: allow using ACPI for RTC alarm
instead of HPET") `rtc-cmos' regression causing a link error:

drivers/rtc/rtc-cmos.o: In function `cmos_platform_probe':
rtc-cmos.c:(.init.text+0x33c): undefined reference to `hpet_rtc_interrupt'
rtc-cmos.c:(.init.text+0x3f4): undefined reference to `hpet_rtc_interrupt'

with non-ACPI platforms using this driver.  The cause is the change of
the condition guarding the use of `hpet_rtc_interrupt'.

Previously it was a call to `is_hpet_enabled'.  That function is static
inline and has a hardcoded 0 result for non-ACPI platforms, which imply
!HPET_EMULATE_RTC.  Consequently the compiler optimized the whole block
away including the reference to `hpet_rtc_interrupt', which never made
it to the link stage.

Now the guarding condition is a call to `use_hpet_alarm', which is not
static inline and therefore the compiler may not be able to prove that
it actually always returns 0 for non-ACPI platforms.  Consequently the
build breaks with an unsatisfied reference, because `hpet_rtc_interrupt'
is nowhere defined at link time.

Fix the problem by marking `use_hpet_alarm' inline.  As the `inline'
keyword serves as an optimization hint rather than a requirement the
compiler is still free to choose whether inlining will be beneficial or
not for ACPI platforms.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Fixes: 311ee9c1 ("rtc: cmos: allow using ACPI for RTC alarm instead of HPET")
Cc: stable@vger.kernel.org # 4.18+
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>