travis: use updated ax_prog_doxygen.m4
travis: use updated ax_prog_doxygen.m4

The version included in the autoconf-archive package by Travis produces
"missing separator" errors when running make distcheck.

Signed-off-by: Jonas Witschel <87a164e5d00d5dcdd001e37b34ea8bcd3fc3a172@gmx.de>

01org/TPM2.0-TSS,01org/tpm2-tss,01org/tpm2-tss,01org/tpm2-tss,tpm2-software/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS,01org/TPM2.0-TSS,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss