[VTA] Infinite recursive device_api.ext_dev call fix. (#3843)

Issue
when try vta on fpga board, would see a Infinite recursive
device_api.ext_dev issue that cause stack overflow and vta
failed.

Analysis:
device_api.ext_dev function in rpc_server.py is use to load
vta library, once vta library get load, device_api.ext_dev would
get replaced with vta function by vta library, vta device_api.cc
did such work, but because a logic issue in VTA.cmake, the said file
not get compiled, then vta would keep failing on rpc_server.py.

Solution:
fix the logic issue in VTA.cmake.

cmake/modules/VTA.cmake

@@ -84,7 +84,7 @@ elseif(PYTHON)
     # Rules for Zynq-class FPGAs with pynq OS support (see pynq.io)
     if(${VTA_TARGET} STREQUAL "pynq" OR
        ${VTA_TARGET} STREQUAL "ultra96")
-      file(GLOB FPGA_RUNTIME_SRCS vta/src/pynq/pynq_driver.cc)
+      list(APPEND FPGA_RUNTIME_SRCS vta/src/pynq/pynq_driver.cc)
     endif()
     # Target lib: vta
     add_library(vta SHARED ${FPGA_RUNTIME_SRCS})