nghttp2: deduplicate files in libnghttp2

libnghttp2 accidentally ships library twice:

$ tar -Oxzf libnghttp2-14_1.38.0-1_mips_24kc.ipk ./data.tar.gz | tar -tzvf -
drwxr-xr-x root/root         0 2019-06-07 23:14 ./
drwxr-xr-x root/root         0 2019-06-07 23:14 ./usr/
drwxr-xr-x root/root         0 2019-06-07 23:14 ./usr/lib/
-rw-r--r-- root/root    144412 2019-06-07 23:14 ./usr/lib/libnghttp2.so.14
-rw-r--r-- root/root    144412 2019-06-07 23:14 ./usr/lib/libnghttp2.so.14.17.3

after fix, there's library and symlink (as designed):

$ tar -Oxzf libnghttp2-14_1.38.0-2_mips_24kc.ipk ./data.tar.gz | tar -tzvf -
drwxr-xr-x root/root         0 2019-06-07 23:14 ./
drwxr-xr-x root/root         0 2019-06-07 23:14 ./usr/
drwxr-xr-x root/root         0 2019-06-07 23:14 ./usr/lib/
lrwxrwxrwx root/root         0 2019-06-07 23:14 ./usr/lib/libnghttp2.so.14 -> libnghttp2.so.14.17.3
-rw-r--r-- root/root    144412 2019-06-07 23:14 ./usr/lib/libnghttp2.so.14.17.3

Binary package size reduced accordingly: 134621 -> 66593.

Compile/run-tested: ar71xx/generic.

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>