Thanks, Hippo!
I removed linux/config.h and am now using linux/semaphore.h. Not getting any more errors about missing header files but still am getting many errors the first few of which are pasted below. Looks like GCC is still looking for things it can't find. "sign_nios_uart" is the custom driver file we wrote.
CC drivers/char/sign_nios_uart.o
drivers/char/sign_nios_uart.c:249: error: 'platform_bus_type' undeclared here (not in a function)
drivers/char/sign_nios_uart.c: In function 'sign_nios_uart_init':
drivers/char/sign_nios_uart.c:268: error: implicit declaration of function 'class_simple_create'
drivers/char/sign_nios_uart.c:268: warning: assignment makes pointer from integer without a cast