Fix 1 missing dependency in Makefile.in

[Vemake Vemake]

Hi, I've fixed 1 missing dependency reported.
This issue can cause incorrect results when gmp is incrementally built.
Detailed speaking, any changes in the target "libtool" will not cause
"dummy.lo" to be rebuilt, which is incorrect.
I've tested it on my computer, the fixed version worked as expected.
The fixed Makefile.in is attached.

Looking forward to your confirmation

Link: https://sourceforge.net/p/softwaremercatinousato/tickets/1/

Thanks
Vemake
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