Adds libm to Pillow recipe (#2465)

Fixes:
```
ImportError: dlopen failed: cannot locate symbol "log" referenced by "_imaging.so"...
```

Author: Sahil-pixel

pythonforandroid/recipes/Pillow/__init__.py

@@ -91,7 +91,7 @@ def get_recipe_env(self, arch=None, with_flags_in_cc=True):
             env['LDFLAGS'] += f' -L{join(webp_install, "lib")}'
         env['LDFLAGS'] += f' -L{ndk_lib_dir}'
         if cflags not in env['CFLAGS']:
-            env['CFLAGS'] += cflags
+            env['CFLAGS'] += cflags + " -lm"
         return env