ltrace: use current master

ltrace 0.7.3 is the latest release but it is actually broken on ARM
since PTRACE_SINGLESTEP emulation has been removed, see:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=425fc47adb5bb69f76285be77a09a3341a30799e



It fails with:
PTRACE_SINGLESTEP: Input/output error

Using master solves that until a new release is made.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>