Tools: Fix Python3 + firmware header traeback

### Description

Traceback:
```
Merging Regions
  Filling region bootloader with mbed-cloud-client-example/mbed-os/features/FEATURE_BOOTLOADER/targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/TARGET_MCU_K64F/TARGET_FRDM/mbed-bootloader-k64f-block_device-sotp-v3_4_0.bin
  Padding region bootloader with 0x9a4 bytes
Traceback (most recent call last):
  File "mbed-cloud-client-example/mbed-os/tools/make.py", line 293, in <module>
    ignore=options.ignore
  File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 548, in build_project
    merge_region_list(region_list, res, notify)
  File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 423, in merge_region_list
    _fill_header(region_list, region).tofile(header_filename, format='hex')
  File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 380, in _fill_header
    header.puts(start, struct.pack(fmt, time()))
struct.error: required argument is not an integer
```

reason: `time()` returns a float. So the fix is to force it to be an `int`.

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change

Author: theotherjimmy

tools/build_api.py

@@ -377,7 +377,7 @@ def _fill_header(region_list, current_region):
         elif type == "timestamp":
             fmt = {"32le": "<L", "64le": "<Q",
                    "32be": ">L", "64be": ">Q"}[subtype]
-            header.puts(start, struct.pack(fmt, time()))
+            header.puts(start, struct.pack(fmt, int(time())))
         elif type == "size":
             fmt = {"32le": "<L", "64le": "<Q",
                    "32be": ">L", "64be": ">Q"}[subtype]