patch 8.2.4038: various code not used when features are disabled

Problem:    Various code not used when features are disabled.
Solution:   Add #ifdefs. (Dominique Pellé, closes #9491)
diff --git a/src/register.c b/src/register.c
index d61a5d0..08a73fe 100644
--- a/src/register.c
+++ b/src/register.c
@@ -38,41 +38,53 @@
 #endif
 static void	dis_msg(char_u *p, int skip_esc);
 
+#if defined(FEAT_VIMINFO) || defined(PROTO)
     yankreg_T *
 get_y_regs(void)
 {
     return y_regs;
 }
+#endif
 
+#if defined(FEAT_CLIPBOARD) || defined(PROTO)
     yankreg_T *
 get_y_register(int reg)
 {
     return &y_regs[reg];
 }
+#endif
 
+#if defined(FEAT_CLIPBOARD) || defined(FEAT_VIMINFO) || defined(FEAT_EVAL) || defined(PROTO)
     yankreg_T *
 get_y_current(void)
 {
     return y_current;
 }
+#endif
 
+#if defined(FEAT_CLIPBOARD) || defined(FEAT_VIMINFO) || defined(PROTO)
     yankreg_T *
 get_y_previous(void)
 {
     return y_previous;
 }
+#endif
 
+#if defined(FEAT_CLIPBOARD) || defined(PROTO)
     void
 set_y_current(yankreg_T *yreg)
 {
     y_current = yreg;
 }
+#endif
 
+#if defined(FEAT_CLIPBOARD) || defined(FEAT_VIMINFO) || defined(PROTO)
     void
 set_y_previous(yankreg_T *yreg)
 {
     y_previous = yreg;
 }
+#endif
 
     void
 reset_y_append(void)
@@ -474,6 +486,7 @@
  */
 static int execreg_lastc = NUL;
 
+#if defined(FEAT_VIMINFO) || defined(PROTO)
     int
 get_execreg_lastc(void)
 {
@@ -485,6 +498,7 @@
 {
     execreg_lastc = lastc;
 }
+#endif
 
 /*
  * When executing a register as a series of ex-commands, if the
@@ -2308,6 +2322,7 @@
     }
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
 /*
  * Return the index of the register "" points to.
  */
@@ -2316,6 +2331,7 @@
 {
     return y_previous == NULL ? -1 : y_previous - &y_regs[0];
 }
+#endif
 
 /*
  * ":dis" and ":registers": Display the contents of the yank registers.