fixed a missing feature

Signed-off-by: Jakub Doka <jakub.doka2@gmail.com>

vm/src/mem/mod.rs

@@ -41,6 +41,7 @@ pub trait Memory {
     fn log_instr(&mut self, _at: Address, _regs: &[Value]) {}
 }
 
+#[cfg(feature = "alloc")]
 #[derive(Default)]
 pub struct InstrLogger {
     #[cfg(debug_assertions)]
@@ -49,6 +50,7 @@ pub struct InstrLogger {
     disp_buf: alloc::string::String,
 }
 
+#[cfg(feature = "alloc")]
 impl InstrLogger {
     /// # Safety
     /// - `addr` needs to point to a valid instruction