fix: guard IMPISPAWBHist declaration for platforms that support it

[themactep]

Problem

IMPISPAWBHist is not defined in T40/T41 SDK headers (only present in T20/T21/T23/T30/T31/C100). The prototype declaration in imp_hal.hpp was unconditional, causing a compile error when building for T41.

Fix

Guard the IMP_ISP_Tuning_GetAwbHist prototype with the same platform condition already used at the call site in imp_hal.cpp:

#if defined(PLATFORM_T20) || defined(PLATFORM_T21) || defined(PLATFORM_T23) || defined(PLATFORM_T30) || \
    defined(PLATFORM_T31) || defined(PLATFORM_C100)
int IMP_ISP_Tuning_GetAwbHist(IMPISPAWBHist *awb_hist);
#endif

Tested

Build verified for vanhua_z66si_t41nq_gc4023_eth (T41 SoC).