ustr: remove source code from target

ustr: remove source code from target

For some reason, ustr installs its own source code, which means we end
up with 448 KB of source code in /usr/share in the target filesystem:

$ tree output/target/usr/share/
output/target/usr/share/
└── ustr-1.0.4
    ├── malloc-check.h
    ├── ustr-b-code.h
    ├── ustr-b-dbg-code.c
    ├── ustr-b-opt-code.c
    ├── ustr-cmp-code.h
    ├── ustr-cmp-dbg-code.c
    ├── ustr-cmp-internal.h
    ├── ustr-cmp-opt-code.c
    ├── ustr-cntl-code.h
    ├── ustr-fmt-code.h
    ├── ustr-fmt-dbg-code.c
    ├── ustr-fmt-internal.h
    [...]

$ du -sh output/target/usr/share/ustr-1.0.4/
448K	output/target/usr/share/ustr-1.0.4/

So let's drop this source code in a post-install target hook.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>