Fix Makefile installplus dependency

[A1ca7raz]

I am packaging this project for Arch Linux RISC-V and I noticed that make install doesn't work on my riscv64 platform.

make: Entering directory '/build/pd-lua/src/pd-lua-0.11.6'
++++ NOTE: using installed lua
++++ info: using Makefile.pdlibbuilder version 0.6.0
++++ info: making target install in lib pdlua
++++ info: installpath is '/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua'
++++info: target all in lib pdlua completed
++++ info: executables of lib pdlua installed from /build/pd-lua/src/pd-lua-0.11.6 to /build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua
install -m 755 -d -v "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua"
++++ info: data files of lib pdlua installed from /build/pd-lua/src/pd-lua-0.11.6 to /build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua
install -m 755 -d -v "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua"
cp -r ./pdlua/ "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua"/pdlua
/usr/bin/install: creating directory '/build/pd-lua/pkg/pd-lua//usr'
/usr/bin/install: creating directory '/build/pd-lua/pkg/pd-lua//usr/lib/pd'
/usr/bin/install: creating directory '/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra'
/usr/bin/install: creating directory '/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua'
/usr/bin/install: creating directory '/build/pd-lua/pkg/pd-lua//usr/lib'
install -p -m 644 'pdlua.pd_linux' "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua";
/usr/bin/cp: cannot create directory '/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua/pdlua': No such file or directory
install -p -m 644 'pd.lua' "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua";  install -p -m 644 'pdlua-help.pd' "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua";  install -p -m 644 'pdluax-help.pd' "/build/pd-lua/pkg/pd-lua//usr/lib/pd/extra/pdlua";
make: *** [Makefile:52: installplus] Error 1
make: *** Waiting for unfinished jobs....
make: Leaving directory '/build/pd-lua/src/pd-lua-0.11.6'

To fix this issue, I edited Makefile to ensure installplus triggers after install-{executables,datafiles,datadirs}, and now it works fine.