sfdpgen: autotools: define NEATOGEN_EXPORTS

sfdpgen: autotools: define NEATOGEN_EXPORTS

sfdpgen is part of the neato_layout plugin.

Fixes these errors 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: ../../lib/sfdpgen/.libs/libsfdpgen_C.a(sfdpinit.o): in function `sfdp_init_node_edge':
C:\Users\magja\graphviz\lib\sfdpgen/sfdpinit.c:37: undefined reference to `__imp_neato_init_node'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/sfdpgen/.libs/libsfdpgen_C.a(sfdpinit.o): in function `tuneControl':
C:\Users\magja\graphviz\lib\sfdpgen/sfdpinit.c:248: undefined reference to `__imp_setSeed'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/sfdpgen/.libs/libsfdpgen_C.a(sfdpinit.o): in function `sfdp_layout':
C:\Users\magja\graphviz\lib\sfdpgen/sfdpinit.c:331: undefined reference to `__imp_spline_edges'
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\lib\sfdpgen/sfdpinit.c:319: undefined reference to `__imp_spline_edges'

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