New review comment by mvf on void-packages repository

https://github.com/void-linux/void-packages/pull/23072#discussion_r676343746

Comment:
This removes a cross-unaware file existence check that breaks the build:
```
checking for /usr/include/qt5/QtWidgets/QWidget... configure: error: cannot check for file existence when cross compiling
=> ERROR: qmidiarp-0.6.5_1: do_configure: '${configure_script} ${configure_args}' exited with 1
=> ERROR:   in do_configure() at common/build-style/gnu-configure.sh:8
```
It's unnecessary too, because the `pkg-config` based check succeeds and returns the same include paths:
```
checking for Qt5Core Qt5Gui Qt5Widgets >= 5.0... yes
```