Missing static for a function in m4f haetae2 implementation (in reduce.h)

[rben-dev]

The m4f haetae2 implementation of pqm4 misses a static keyword in

pqm4/crypto_sign/haetae2/m4f/reduce.h

Line 18 in 8d44b72

inline int32_t _montgomery_reduce(int64_t a) {

(the other variants are not impacted since they use an implementation of the function in the .c file). This can produce compilation errors (missing symbol) in some cases where the compiler decides to not inline the function in compilation units.