chore: add build file to mempool_node to allow usage of starknet_sier…

…ra_compile crate (#1126)

crates/mempool_node/build.rs

@@ -0,0 +1,3 @@
+// Sets up the environment variable OUT_DIR, which holds the cairo compiler binary.
+// The binary is dowloaded to OUT_DIR by the starknet_sierra_compile crate.
+fn main() {}