mfd: tps6594: Explicitly include bitfield.h

After a recent change that started using FIELD_GET() in tps6594-core.c,
there is an error when bitfield.h is not implicitly included, such as
when building allmodconfig for ARCH=hexagon:

drivers/mfd/tps6594-core.c:767:7: error: call to undeclared function 'FIELD_GET'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
767 | if (FIELD_GET(TPS65224_MASK_EN_PB_VSENSE_CONFIG, pwr_on) == TPS65224_EN_SEL_PB ||
| ^

Explicitly include bitfield.h to resolve the errors.

Reported-by: kernel test robot <[e-mail]>
Fixes: d766ca01c208 ("mfd: tps6594: Add power button functionality")
Signed-off-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Michael Walle <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/tps6594-core.c

···
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  */
 
+#include <linux/bitfield.h>
 #include <linux/completion.h>
 #include <linux/delay.h>
 #include <linux/interrupt.h>