arm64: fix build without FDT

static_kenv is only used under `FDT`, and `try_load_dtb` is only defined
with `FDT`.

Reviewed by:	andrew, imp, manu
Differential Revision:	https://reviews.freebsd.org/D36791

sys/arm64/arm64/machdep.c

@@ -768,7 +768,9 @@ initarm(struct arm64_bootparams *abp)
 	update_special_regs(0);
 
 	link_elf_ireloc(kmdp);
+#ifdef FDT
 	try_load_dtb(kmdp);
+#endif
 
 	efi_systbl_phys = MD_FETCH(kmdp, MODINFOMD_FW_HANDLE, vm_paddr_t);
 

sys/arm64/arm64/machdep_boot.c

@@ -53,9 +53,10 @@ __FBSDID("$FreeBSD$");
 
 extern int *end;
 static char *loader_envp;
-static char static_kenv[4096];
 
 #ifdef FDT
+static char static_kenv[4096];
+
 #define	CMDLINE_GUARD "FreeBSD:"
 #define	LBABI_MAX_COMMAND_LINE 512
 static char linux_command_line[LBABI_MAX_COMMAND_LINE + 1];