Bug 1857190 - include alloca.h on Solaris

diff --git a/dist/karamel/include/krml/internal/builtin.h b/dist/karamel/include/krml/internal/builtin.h
index f55e5f824e..07ff156788 100644
--- a/dist/karamel/include/krml/internal/builtin.h
+++ b/dist/karamel/include/krml/internal/builtin.h
@@ -7,6 +7,8 @@
 /* For alloca, when using KaRaMeL's -falloca */
 #if (defined(_WIN32) || defined(_WIN64))
 #include <malloc.h>
+#elif (defined(sun))
+#include <alloca.h>
 #endif
 
 /* If some globals need to be initialized before the main, then karamel will