MAINTAINERS: update entry to thermal governors file name prefixing

Commit 0015d9a2 ("thermal/governors: Prefix all source files with
gov_") renamed power_allocator.c to gov_power_allocator.c in
./drivers/thermal amongst some other file renames, but missed to adjust
the MAINTAINERS entry.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/thermal/power_allocator.c

Update the file entry in MAINTAINERS to the new file name.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200728045850.22661-1-lukas.bulwahn@gmail.com

MAINTAINERS

@@ -16968,7 +16968,7 @@ M:	Lukasz Luba <lukasz.luba@arm.com>
 L:	linux-pm@vger.kernel.org
 S:	Maintained
 F:	Documentation/driver-api/thermal/power_allocator.rst
-F:	drivers/thermal/power_allocator.c
+F:	drivers/thermal/gov_power_allocator.c
 F:	include/trace/events/thermal_power_allocator.h
 
 THINKPAD ACPI EXTRAS DRIVER