[platform/target][warnings] fix -Wmissing-declarations warnings in platform/ and target/

Mostly driver code in various platforms. There are still some warnings
in this part of the tree in lesser-used platforms.

target/init.c

@@ -21,9 +21,6 @@ __WEAK void target_early_init(void) {
 __WEAK void target_init(void) {
 }
 
-__WEAK void target_set_led(unsigned int led, bool on) {
-}
-
 __WEAK void target_quiesce(void) {
 }
 

target/pc-x86/config.c

@@ -13,6 +13,7 @@
 #include <platform/ide.h>
 #include <platform/pcnet.h>
 #include <platform.h>
+#include <target.h>
 #include <malloc.h>
 #include <string.h>
 #include <lk/debug.h>

target/uzed/target.c

@@ -12,6 +12,7 @@
 #include <kernel/vm.h>
 #include <platform/zynq.h>
 #include <platform/gem.h>
+#include <target.h>
 
 zynq_pll_cfg_tree_t zynq_pll_cfg = {
     .arm = {

target/zybo/target.c

@@ -15,6 +15,7 @@
 #include <platform/gem.h>
 #include <platform/gpio.h>
 #include <platform/interrupts.h>
+#include <target.h>
 #include <target/gpioconfig.h>
 
 zynq_pll_cfg_tree_t zynq_pll_cfg = {