misc: fastrpc: select CONFIG_DMA_SHARED_BUFFER

Fastrpc is a dma buf exporter as well, so select the corresponding
DMA_SHARED_BUFFER config to fix below compilation errors on platforms
without this config.

ld: drivers/misc/fastrpc.o: in function 'fastrpc_free_map':
fastrpc.c:(.text+0xbe): undefined reference to 'dma_buf_unmap_attachment'
ld: fastrpc.c:(.text+0xcb): undefined reference to 'dma_buf_detach'
ld: fastrpc.c:(.text+0xd4): undefined reference to 'dma_buf_put'
ld: drivers/misc/fastrpc.o: in function 'fastrpc_map_create':
fastrpc.c:(.text+0xb2b): undefined reference to 'dma_buf_get'
ld: fastrpc.c:(.text+0xb47): undefined reference to 'dma_buf_attach'
ld: fastrpc.c:(.text+0xb61): undefined reference to 'dma_buf_map_attachment'
ld: fastrpc.c:(.text+0xc36): undefined reference to 'dma_buf_put'
ld: fastrpc.c:(.text+0xc48): undefined reference to 'dma_buf_detach'
ld: drivers/misc/fastrpc.o: in function 'fastrpc_device_ioctl':
fastrpc.c:(.text+0x1756): undefined reference to 'dma_buf_get'
ld: fastrpc.c:(.text+0x1776): undefined reference to 'dma_buf_put'
ld: fastrpc.c:(.text+0x1780): undefined reference to 'dma_buf_put'
ld: fastrpc.c:(.text+0x1abf): undefined reference to 'dma_buf_export'
ld: fastrpc.c:(.text+0x1ae7): undefined reference to 'dma_buf_fd'
ld: fastrpc.c:(.text+0x1cb5): undefined reference to 'dma_buf_put'
ld: fastrpc.c:(.text+0x1cca): undefined reference to 'dma_buf_put'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>