Missing `ifdef guard from assertion in ibex_core.sv

Nobody has claimed this yet.

## Observed Behavior

Building ibex without INC_ASSERT define fails due to missing guard in [ibex_core.sv](https://github.com/lowRISC/ibex/blob/master/rtl/ibex_core.sv#L1858).

## Expected Behavior

Expecting core to elaborate without this define.

## Steps to reproduce the issue

This assertion relies on logic only present in [ibex_ex_block.sv](https://github.com/lowRISC/ibex/blob/master/rtl/ibex_ex_block.sv) when it is compiled with this assertion. If ex_block is compiled with the define, elaboration succeeds without issues.

## My Environment

RHEL7 / Questa

Compiling source codes based on external filelists and Questa commands in makefile.

**Version of the Ibex source code:**

The issue is present in current master (53888bcdf4ca3c07e5e715fb6386cb4cc643a61b), assertion was introduced by [[rtl] Guard...](https://github.com/lowRISC/ibex/blame/master/rtl/ibex_core.sv#L1858) merge.

## Edit: Testing more locally

This seems to be more complex issue than I originally figured. Simply inserting guard ifdef does not fix the issue, Module seems to be inheriting the INC_ASSERT define from somewhere (and the ex_block does not). Probably from includes that define the macros, but I did not investigate that too much yet.

Alternative is to use the SYNTHESIS define which of course disables all assertions.

Start at the assertion around line 1858 of rtl/ibex_core.sv and compare how INC_ASSERT is defined when rtl/ibex_ex_block.sv is compiled. Reproduce the failure with the external filelists and Questa commands described in the issue, then trace the relevant includes and defines. Done means the core elaborates without INC_ASSERT while preserving assertion behavior when the define is enabled.

Written by the indexing model from the issue text.