[klibc] zlib: Suppress implicit-fallthrough warning

zlib implements state machines with for and switch, but some state
transitions are "optimised" as a fall-through between switch cases.
Rather than deviating from upstream zlib, suppress the warnings for
these.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
diff --git a/usr/klibc/zlib/Kbuild b/usr/klibc/zlib/Kbuild
index f2e1bf2..8bff3d5 100644
--- a/usr/klibc/zlib/Kbuild
+++ b/usr/klibc/zlib/Kbuild
@@ -6,3 +6,7 @@
 
 # zlib specific flag
 EXTRA_KLIBCCFLAGS := -DDYNAMIC_CRC_TABLE
+
+# Suppress implicit-fallthrough warning for state machine cases
+# where this is intentional
+EXTRA_KLIBCCFLAGS += $(call cc-disable-warning,implicit-fallthrough)