build: point to directory with overriden tools

mpv overrides waf tool "syms" found in "waftools" directory, but  when building with system waf, this overriden version is not picked up which results in error:
```
Build failed
Traceback (most recent call last):
  File "/usr/share/waf/waflib/Task.py", line 241, in process
    ret = self.run()
  File "/usr/share/waf/waflib/Task.py", line 85, in run
    return m1(self)
  File "/usr/share/waf/waflib/extras/syms.py", line 40, in run
    re_nm = re.compile(r'T\s+(' + self.generator.export_symbols_regex + r')\b')
AttributeError: 'task_gen' object has no attribute 'export_symbols_regex'
```
Error is only present when building with shared client lib enabled by --enable-libmpv-shared. Please apply patch which explicitly points to tool directory.