algorithm.cpp: Add initializer to pacify gcc

[fbleibel-g]

Before this change, make test gives:

g++ -I. -c -o obj/test/algorithm.o test/algorithm.cpp  -O2 -ffast-math -fstrict-aliasing -fPIE  -std=c++14 -Wall
test/algorithm.cpp: In function ‘void nda::test_transform_index_body()’:
test/algorithm.cpp:125:34: error: call of overloaded ‘array(<brace-enclosed initializer list>)’ is ambiguous
  125 |   dense_array<int, 1> a_1d({1024});