unity builds: correct integer ceiling division

* edge case failure with unity builds:
  - static archive bar that gets installed, that links with another static
    archive foo that does not get installed
  - the number of files in static archive foo is divisible by unity_size

would yield an error with ninja:

  ninja: error: 'subprojects/foo/src/libfoo.a.p/meson-generated_foo-unity1.cpp.o', needed by 'src/libbar.a', missing and no known rule to make it

mesonbuild/backend/backends.py

@@ -920,7 +920,7 @@ def _determine_ext_objs(self, extobj: 'build.ExtractedObjects', proj_dir_to_buil
                 if comp.language in LANGS_CANT_UNITY:
                     sources += srcs
                     continue
-                for i in range(len(srcs) // unity_size + 1):
+                for i in range((len(srcs) + unity_size - 1) // unity_size):
                     _src = self.get_unity_source_file(extobj.target,
                                                       comp.get_default_suffix(), i)
                     sources.append(_src)