Fix symbol issue in Vivado 2018

[nekomona]

Fix the following error in Vivado 2018.3 and newer versions

create_bd_cell -type ip -vlnv clifford.at:user:picorv32:1.0 picorv32_0
WARNING: [IP_Flow 19-3331] Invalid parameters found in dependency constraints of 'ADDRSPACE_WIDTH.M_AXI' 
WARNING: [IP_Flow 19-3331] Invalid parameters found in dependency constraints of 'ADDRSPACE_WIDTH.M_AXI' 
WARNING: [IP_Flow 19-3553] Error found in parameter dependencies in XGUI files. Invalid parameters found in dependency constraints of 'ADDRSPACE_WIDTH.M_AXI' 

ERROR: [IP_Flow 19-3428] Failed to create Customization object design_1_picorv32_0_0
CRITICAL WARNING: [IP_Flow 19-973] Failed to create IP instance 'design_1_picorv32_0_0'. Error during customization.
ERROR: [BD 41-1712] Create IP failed with errors
ERROR: [BD 5-7] Error: running create_bd_cell  -vlnv clifford.at:user:picorv32:1.0 -type ip -name picorv32_0 .
ERROR: [Common 17-39] 'create_bd_cell' failed due to earlier errors.

[nekomona]

Solving the issue in #1