Adds libm to Pillow recipe (kivy#2465)

Sahil-pixel · web-flow · commit 4b1c7d8a7557 · 2021-06-17T23:31:27.000+02:00
Fixes:
```
ImportError: dlopen failed: cannot locate symbol "log" referenced by "_imaging.so"...
```
diff --git a/pythonforandroid/recipes/Pillow/__init__.py b/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
 
 
