mapi: Massage code to allow clang to compile.

According to https://llvm.org/bugs/show_bug.cgi?id=19778#c3 this code
was violating the spec, resulting in it failing to compile.

Cc: mesa-stable@lists.freedesktop.org
Co-authored-by: Tomasz Paweł Gajc <tpgxyz@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89599
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index 3799d8d..1ca8359 100644
--- a/configure.ac
+++ b/configure.ac
@@ -226,6 +226,7 @@
 AX_GCC_FUNC_ATTRIBUTE([pure])
 AX_GCC_FUNC_ATTRIBUTE([returns_nonnull])
 AX_GCC_FUNC_ATTRIBUTE([unused])
+AX_GCC_FUNC_ATTRIBUTE([visibility])
 AX_GCC_FUNC_ATTRIBUTE([warn_unused_result])
 AX_GCC_FUNC_ATTRIBUTE([weak])