selftests: riscv: Fix compilation error with vstate_exec_nolibc.c

selftests: riscv: Fix compilation error with vstate_exec_nolibc.c

The following error happens:

In file included from vstate_exec_nolibc.c:2:
/usr/include/riscv64-linux-gnu/sys/prctl.h:42:12: error: conflicting types for ‘prctl’; h
ave ‘int(int, ...)’
   42 | extern int prctl (int __option, ...) __THROW;
      |            ^~~~~
In file included from ./../../../../include/nolibc/nolibc.h:99,
                 from <command-line>:
./../../../../include/nolibc/sys.h:892:5: note: previous definition of ‘prctl’ with type
‘int(int,  long unsigned int,  long unsigned int,  long unsigned int,  long unsigned int)
’
  892 | int prctl(int option, unsigned long arg2, unsigned long arg3,
      |     ^~~~~

Fix this by not including <sys/prctl.h>, which is not needed here since
prctl syscall is directly called using its number.

Fixes: 7cf6198ce22d ("selftests: Test RISC-V Vector prctl interface")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230713115829.110421-1-alexghiti@rivosinc.com
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>