pixman-combine-float.c: fix inlining failed error

Refer [1], always-inline is not suggested to be used if you have indirect
calls. so replace force_inline with inline to fix error like:
In function ‘combine_inner’,
    inlined from ‘combine_soft_light_ca_float’ at ../pixman/pixman-combine-float.c:655:511:
../pixman/pixman-combine-float.c:655:211: error: inlining failed in call to ‘always_inline’ ‘combine_soft_light_c’: function not considered for inlining

Test with gcc-9 and gcc-14, both works well

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115679



Signed-off-by: Changqing Li <changqing.li@windriver.com>