powerpc/mpc5200: include fs.h in mpc52xx_gpt.c

Fix build errors like these (from a randconfig and my defconfig for a custom board):

src/arch/powerpc/platforms/52xx/mpc52xx_gpt.c:549: error: dereferencing pointer to incomplete type: 1 errors in 1 logs
src/arch/powerpc/platforms/52xx/mpc52xx_gpt.c:636: error: implicit declaration of function 'nonseekable_open': 1 errors in 1 logs
src/arch/powerpc/platforms/52xx/mpc52xx_gpt.c:657: error: variable 'mpc52xx_wdt_fops' has initializer but incomplete type: 1 errors in 1 logs
src/arch/powerpc/platforms/52xx/mpc52xx_gpt.c:658: error: excess elements in struct initializer: 1 errors in 1 logs
src/arch/powerpc/platforms/52xx/mpc52xx_gpt.c:658: error: unknown field 'owner' specified in initializer: 1 errors in 1 logs
...

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>