Add the sketch folder as the first include search path (#8734)

This allow for example to have LVGL's config in the sketch folder and it to be project specific this way.

Author: me-no-dev

platform.txt

@@ -49,7 +49,7 @@ compiler.warning_flags.more=-Wall -Werror=all
 compiler.warning_flags.all=-Wall -Werror=all -Wextra
 
 # Compile Flags
-compiler.cpreprocessor.flags="@{compiler.sdk.path}/flags/defines" -iprefix "{compiler.sdk.path}/include/" "@{compiler.sdk.path}/flags/includes" "-I{compiler.sdk.path}/{build.memory_type}/include"
+compiler.cpreprocessor.flags="@{compiler.sdk.path}/flags/defines" "-I{build.source.path}" -iprefix "{compiler.sdk.path}/include/" "@{compiler.sdk.path}/flags/includes" "-I{compiler.sdk.path}/{build.memory_type}/include"
 compiler.c.flags="@{compiler.sdk.path}/flags/c_flags" {compiler.warning_flags} {compiler.optimization_flags}
 compiler.cpp.flags="@{compiler.sdk.path}/flags/cpp_flags" {compiler.warning_flags} {compiler.optimization_flags}
 compiler.S.flags="@{compiler.sdk.path}/flags/S_flags" {compiler.warning_flags} {compiler.optimization_flags}