doc: raise doxygen lookup cache
At the end of a run, doxygen emits a notice regarding the cache lookup
size being too small. This cache is used to find symbols given their
name which would speed up processing a bit.
The notice before this patch:
lookup cache used 65536/65536 hits=
2258467 misses=112595
Note: based on cache misses the ideal setting for LOOKUP_CACHE_SIZE is 1
at the cost of higher memory usage.
And after:
lookup cache used 104632/131072 hits=
2131285 misses=105434
That is 7% less miss, not that much but still. Note how the cache is
well filled (80%).
Change-Id: I0b97e8c5581a63c44a382891f8f1fc8d60975e80