neatogen: add storage-class attribute declarations in neatoprocs.h

neatogen: add storage-class attribute declarations in neatoprocs.h

Fixes this error using autotools with MinGW:
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: gvpack-gvpack.o: in function `init_node_edge':
C:\Users\magja\graphviz\cmd\tools/gvpack.c:271: undefined reference to `neato_init_node'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\magja\graphviz\cmd\tools/gvpack.c:272: undefined reference to `user_pos'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: gvpack-gvpack.o: in function `init_graph':
C:\Users\magja\graphviz\cmd\tools/gvpack.c:305: undefined reference to `init_nop'

Towards https://gitlab.com/graphviz/graphviz/-/issues/2058