chore: fix a compilation bug on MacOS and update .gitignore

[xjtuwxg]

1. Got the following compilation error on MacOS 15.5 (x86_64-elf-gcc (GCC) 15.1.0). It seems because nullptr becomes a keyword in C++11 and later.

make
rm -f ".depend"
x86_64-elf-gcc -MM bio.c bootmain.c cat.c console.c echo.c exec.c file.c forktest.c fs.c grep.c ide.c init.c ioapic.c kalloc.c kbd.c kill.c lapic.c ln.c log.c ls.c main.c memide.c mkdir.c mkfs.c mp.c pipe.c printf.c proc.c rm.c sh.c sleeplock.c spinlock.c stressfs.c string.c syscall.c sysfile.c sysproc.c trap.c uart.c ulib.c umalloc.c usertests.c vm.c wc.c zombie.c > ".depend"
x86_64-elf-gcc -fno-pic -static -fno-builtin -fno-strict-aliasing -Wall -MD -ggdb -fno-omit-frame-pointer -ffreestanding -fno-common -nostdlib -m64 -DX64 -mcmodel=large -mtls-direct-seg-refs -mno-red-zone -O0 -fno-stack-protector   -c -o usertests.o usertests.c
usertests.c:285:1: error: expected identifier or '(' before 'nullptr'
  285 | nullptr(void)
      | ^~~~~~~
usertests.c: In function 'main':
usertests.c:1651:3: error: called object is not a function or function pointer
 1651 |   nullptr();
      |   ^~~~~~~
make: *** [usertests.o] Error 1

2. It seems that .depend should be added to .gitignore.