regulators: db8500: Fix compile failure for drivers/regulator/dbx500-prcmu.c

Building for the snowball board, I ran into this compile failure:

  CC      drivers/regulator/dbx500-prcmu.o
arm-test.git/drivers/regulator/dbx500-prcmu.c:119:11: error: 'THIS_MODULE' undeclared here (not in a function)
make[3]: *** [drivers/regulator/dbx500-prcmu.o] Error 1
make[2]: *** [drivers/regulator] Error 2

Commit 38e96838 "regulators/db8500: split off shared dbx500 code"
separated out the dbx500 code but did not copy over the required include
to linux/module.h.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>