src/exec_ptrace: fix build without precess_vm_readv()

Commit 32f4b98 (sudo frontend: silence most -Wconversion warnings.)
broke the build on C libraries that miss process_vm_readv(), like
uClibc-ng.

Indeed, the ssize_t nwritten is declared guarded by HAVE_PROCESS_VM_READV,
but is then re-assigned and used a few lines below, outside any guard.

Fix that by always declaring the object, as it is always needed.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

src/exec_ptrace.c

@@ -878,11 +878,12 @@ ptrace_write_vec(pid_t pid, struct sudo_ptrace_regs *regs, char **vec,
     unsigned long addr, unsigned long strtab)
 {
     const unsigned long strtab0 = strtab;
+    ssize_t nwritten;
     size_t i;
     debug_decl(ptrace_write_vec, SUDO_DEBUG_EXEC);
 
 #ifdef HAVE_PROCESS_VM_READV
-    ssize_t nwritten = ptrace_writev_vec(pid, regs, vec, addr, strtab);
+    nwritten = ptrace_writev_vec(pid, regs, vec, addr, strtab);
     if (nwritten != -1 || errno != ENOSYS)
 	debug_return_ssize_t(nwritten);
 #endif /* HAVE_PROCESS_VM_READV */