[libFuzzer] Avoid fuzzer symbols being hidden.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1490845#c5.

Patch By: Mike Hommey

llvm-svn: 342423
GitOrigin-RevId: 1e1f3c8298d2992b0157f60fcd5c44a5ec13b446
diff --git a/FuzzerExtFunctionsWeak.cpp b/FuzzerExtFunctionsWeak.cpp
index a4e56fc..6a6ef49 100644
--- a/FuzzerExtFunctionsWeak.cpp
+++ b/FuzzerExtFunctionsWeak.cpp
@@ -22,7 +22,7 @@
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
-  __attribute__((weak)) RETURN_TYPE NAME FUNC_SIG
+  __attribute__((weak, visibility("default"))) RETURN_TYPE NAME FUNC_SIG
 
 #include "FuzzerExtFunctions.def"