libsel4debug: Guard functions using cap identify

The cap identification function only exists on builds when the kernel is
in debug mode. In release, this function doesn't exist and the compiler
raises warnings. This commit adds the preprocessor guards to avoid
compiling these functions when the cap identification function doesn't
exist.

Signed-off-by: Damon Lee <Damon.Lee@data61.csiro.au>

libsel4debug/include/sel4debug/debug.h

diff --git a/libsel4debug/include/sel4debug/debug.h b/libsel4debug/include/sel4debug/debug.h
index 8803f8b..ad0f2bc 100644
--- a/libsel4debug/include/sel4debug/debug.h
+++ b/libsel4debug/include/sel4debug/debug.h
@@ -21,6 +21,7 @@
 
 void debug_cap_identify(seL4_CPtr cap);
 
+#ifdef CONFIG_DEBUG_BUILD
 static inline int debug_cap_is_valid(seL4_CPtr cap)
 {
     return (0 != seL4_DebugCapIdentify(cap));
@@ -39,7 +40,7 @@
     // cap_notification_cap = 6, but we can't include it here
     return (6 == seL4_DebugCapIdentify(cap));
 }
-
+#endif /* CONFIG_DEBUG_BUILD */
 
 void debug_print_bootinfo(seL4_BootInfo *info);